surfasb essentially has the right idea. Once you have one virus, it's already downloaded all its virus friends and they download updates to avoid virus scanners.
You can try to remove the viruses but you'll never know if you really got rid of all of them or if there are still some malicious settings left over.
The best way to know there are no more viruses is to reinstall.
If you still want to try to remove the viruses, you can try this:
There is some pretty good information here that you can try.
short version:
Leave the computer disconnected from the internet for a week to keep the viruses from updating. Take the infected hard drive out of the computer and hooked it up to a known good computer as a secondary drive. Update virus definitions on your clean system and scan the infected hard drive. Most antivirus products have an option to do a custom scan which would let you just scan the attached (infected) drive.
I've had good luck with SuperAntiSpyware. There is a free version. Worth a shot. The more antivirus programs you scan with, the better.
Stick the previously infected drive back in the computer, re-install antivirus (it might have been crippled). Make sure you have all the latest windows updates and service packs.
So that is going to leave my machine in its current state? Current applications and settings etc? – peter – 2011-07-30T00:39:08.633
The short answer is no. It's going to break a lot of programs. That is how you know for sure any viruses/malware will have their connection to the OS broken. The good thing about going this route, is all your old data will be in a folder called Windows.old. At this point, it is impossible to tell what is a legitimate application and what is malware and I would not bet your data on it. While it sucks, identity theft sucks more and takes even longer to fix. – surfasb – 2011-07-30T01:32:25.427
The below answer is also worth a try. Lesson for the future. Make backups. Harddrives are dirt cheap these days. The amount people pay for cable TV in a month will buy you MORE than enough harddrive space for 6 months worth of daily backups. If you aren't confident that you won't get a virus again, you gotta prepare yourself for the inevitable. – surfasb – 2011-07-30T01:34:52.697
I do have backups, so I don't think that is too much of an issue. It is just weighing up whether I can fix this, or whether I should upgrade like you say, or perhaps even install fresh on a new hard drive. Can malware install itself into things like photos? From my backups my photos are the most important thing, most of the other stuff I can purge. – peter – 2011-07-30T01:38:45.050
Another thought, if I install fresh on a new hard drive, and keep the old hard drive to copy over some of my data would malware spread like that if I am careful? – peter – 2011-07-30T01:41:33.037
1You'll be ok. General rule is malware/viruses are activated by running a program. Which is why it is always recommended that you reinstall your programs from a trusted source, like a setup dvd rather than draggin them over from the old installation. For all we know, a virus changed up Microsoft Word and makes it nuke all your files everytime it runs. . . – surfasb – 2011-07-30T01:46:12.690