[Unknown_Sniper]

I have been having this weird problem with my computer. About a week ago I noticed my computer running way to slow. I hit ctrl+alt+delete and noticed that one of my svchost.exe processes was taking up 95%+ of my processing. I stoped it and my computer started to shut down. did the little shutdown - a thing to turn it off and bam my computer worked and wasnt screwed up for about half a week. Last night it came back. Now when I get rid of the Svchost I cant open up and desktop files, copy and paste links or text of any sort and I cant use and media files. All I can do is semi browse the web and play solitare.
How can I fix this without having to reformat yet again? Also what could be causing this? I did virus scans and they come up neg, get rid of spyware and all that and it still happens. Only one Svchost.exe process is doing this, the others are fine.