A number of viruses can do either together or single handedly.
My suggestion is you run the "Msconfig" command and do a selective startup. disable all the programs from starting up as the viruses sometimes hide behind them.
apply and restart. you will see it get slightly faster on startup.
Then depending on how the infection was done, you can run Windows repair so that you replace all the windows applications that don't start up.
Get a good antivirus, i wd go for F-secure or Sophos in this case, install and update it, then run a thorough scan, it wills show you the virus as well as
the infected file.
After you know the virus, you can deal with it. Its true some viruses may not antivirus solutions, but atleast F-secure and sophos, will tell you what a
virus does, where is resides, what it writes to the registry and the services it corrupts.
You will be able to manually remove these registry entries, delete the file say in safe mode and you will be up and running.
You can also run a registry cleaning utility to crown it all
FORMATTING SHOULD BE YOUR LAST, OPTION, unless you have been backing up, which I am sure u haven't and you can't do that now coz you will only backup viruses.