I can not see what you have so you have to tell us so we can help you here.
Have you any working up to date AV product loaded on this system?
Have you scanned for infections?
Have you changed anything prior to this happening?
Was there anything strange occurring before this started?
Generally speaking every time I have seen this it was a Infection of some sought where the user either failed to have a working AV product in place or didn't regularly scan to prevent infections taking control. And in one case where the user couldn't help themselves if they saw something on the screen they just had to click on it no matter what it was so they OK disabling the AV Product and installed the Virus without any understanding of what it was that they where doing.
The only easy fast solution I have found for this is to remove the HDD fit it to an External USB Caddy and backup the Data off it. Hopefully if you where using EFS you have backed up the Recovery Agent because if you haven't your Data will be lost forever as you [b]Can Not[/b] remove the encryption under anything but the working OS with the Recovery Agent Reloaded.
After you have backed up your Data use a Utility like Boot & Nuke to wipe the HDD
http://www.dban.org/
Then when that is finished load the system from either the Recovery Set or the Install Disc's that where provided with the computer new. Once you have it reloaded you can move your data back and continue working.
There are however some steps that you may be required to take depending on which version of Windows you have installed. Things like for XP 32 Bit you need to install any Realtech Sound Drivers before connecting tot he Windows Update Servers.
Col