If the other suggestions dont help, try the following. It has...

If the other suggestions dont help, try the following. It has worked for me on several systems.

Sounds like a trojan.
Do you have any AV installed currently?
Do you still have internet access?

A few things to try;

1. First you need to identify the virus. There are some virus removal utilities available which might save you a lot of messing around.
2. Disable System Restore
3. Download & run adaware/spybot etc. It's likely you won't be able to run these with the virus active.
4. I usually find it easier to remove the hard drive & scan it on another system with updated AV.
5. After the scan on the other system is complete, plug it back into your system & run adaware/spybot, reboot & then do a full system scan for virus's.

This should get rid of it.

Alternatively, format & re-load if the above sounds too daunting.

Good luck.