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.
