Here is a nice list and review of a few Portable Antivirus Software, which are freeware to use.
http://thepcsecurity.com/ultimate-list-of-portable-antiviruses-for-your-usb/

Whilst it hasn't got the best detection rate of all the anti virus programs it is one I always have with me on my USB stick for dealing with infected machines.

This program suits my needs for something I can run if I sense a problem and otherwise just have available but not running (to save CPU and RAM use). The only warning I have is to not enable automatic quarantine of infected files. This could render your computer non-functional if system files are auto-quarantined. While this might be desired in some circumstances, probably the average user would prefer not to reboot into a broken-down and therefore locked system.

Simply turning this option off means that you can still discover malware infections and then deal with cleaning them up manually after you find out how bad the infection is. This is a preferred type of approach in this modern age of false positives.

ClamWin is nice though. I like the simple interface and ability to exclude certain media playback filetypes that might be considered to large and not dangerous enough to need to scan.

I like how it doesn't install a bunch of strange background processes.

Thanks to the author of this program and for keeping it up to date.

this is not a very good antivirus app.it failed on some viruses for me while others did the job like nod32 or etrust
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tuyen dung | viec lam | tim viec

@SomeGuy

It's based on ClamAV, hence the name "ClamWin"

It is slow, but it is open source and i have heard it is based on AVG, because of this, there will be less bugs and you know that what you downloaded isnt a virus itself because you can see its source

It's always worked well for me. I use it on a write-protected USB stick when diagnosing systems.
Definitely better than Symantec or AVG in that it doesn't quarantine USEFUL tools like netcat.

I'm unable to load the preference dialog box, which makes scanning take forever.

Great Open Source app, but a damm shame its so slow!

Vista Compatible (Vista Ultimate) no UAC warnings.
Portable DB Updater also works but does have UAC Warnings

Is it http://www.clamwin.com/content/view/118/89/ stealth??

I can't say how good the protection is for this is.
It's never picked up a virus upon my pc, but then again neither has norton mcafee or avg.

Two problems I have with it though is it's getting HUGE.
I've had to take it off one of my usb sticks because there was no room for the definitions.

Also it's soooooooooooooooooooooo slow.
I left it to scan my laptop, came back hours later and there it was still halfway trough my i386 folder.

It seems well put together and the volunteers eady to help but at this speed I'm looking for something else.

I have been trying since version 0.88.5 and I still can't get the program to work because "ClamWin Antivirus has encountered a problem and needs to close". I have tried it on 3 different computers and about 7 different hard drives and the result is still the same. I can't get anyone at the ClamWin Portable forum to comment on my problem so maybe someone here will.

UPDATE:

you can make the update window automatically close upon completion if you instead put the following in the batch file:

"start /wait ClamWinPortable.exe --mode=update --close
start ClamWinPortable.exe"

That might be more helpful, as you don't then have to wait for the definition update to complete and then press "close."

Regarding auto-updates of the virus definitions, there are some good postings at the portableapps.com forums.

Specifically:

http://portableapps.com/node/2920 for constantly keeping your definitions up to date (re: ClamWinPortableDBUpdate)

http://portableapps.com/node/2909 points out that if you don't need your definitions CONSTANTLY up to date, you can simply create a batch file with the following, in order to make ClamWin update before launch:

"start /wait ClamWinPortable.exe --mode=update
start ClamWinPortable.exe"

(note that the original portableapps.com forum has the WRONG name for the executable, use the above text instead)

I agree that it may not be as good as some of the other virus scanners. I use it as a great portable tool to analyze customer pc's as an initial sweep. I then get them started with the other installed programs.

Keep up the great work on the site. I direct many of my students this way every semester.

I use it on my thumb drive when traveling + At home use it as part of Spyware Terminator.

Hey guys! It's a FREEWARE antivirus! Please be more compressive... You can't comparate it with Nod, Kaspersky, etc...
Just try it if you need it, if not, keep using Nod ;)

Since updating from V0.90 to V0.90.1.1 I've problems to update the signatures. I get a messege that main db is up to date but getting the daily on ALL servers fails!

I'm afraid my opinion of this is rubbish! No matter what version I use (since Nov '06) I have NEVER been able to scan anything because as soon as I press the Scan button ClamWin crashes.

as the website states, it's opensource, and updated by volunteers...so, if you notice something that isn't detected, tell them about it; it only makes the project better.

I'm glad it doesn't rely on hueristics that other engines rely on; results in waaaaaaayyy too many false positives and other crap, that scare the user into thinking they are actually at risk for an infection; when in reality it's just a fake file or whatever.

agreed. the app is not good enough. too slow and also the virus base is not full.

this is not a very good antivirus app.it failed on some viruses for me while others did the job like nod32 or etrust.

If you use this inconjunction with winpooch you'll have yourself a free proactive virus scanner.