Cannot install anti-spyware/trojan scanners-HELP

Hello, I am very new to this forum but am absolutely desperate for some help-
My compter is ifected with (one or more) trojans and spyware, and it will not let me install any anti spyware.
Antivir Guard and Microsoft Anrispyware turn up nothing when the system is scanned and when I try to install new scanners the installer always freezes on a file with an .lnk tag.
A programme called svschost.exe is also active on my computer and I can't get to it.
My computer will not let me into the 'add and remove software' bit of my control panel either....
I am pretty stuck.
How do I get rid of all this?
PLEASE HELP!

Cheers, Marlous

 

according to Sophos, svschost.exe belongs to Worm/Sdbot . Try this in safe mode : http://download.drweb.com/drweb cureit/

 

Yeah, did that, but that didn't get rid of the trojans etc...
Scanned my system with several other cure-it-alls, but they could only fix my problems if I actually bought them (SpyDoctor and the like). As I am kind of under pressure for time and extremely skint, could you point me in the right direction in fixing this? My last resort is just backing up everything and formatting my computer, but if I don't have to I'd rather not.
Cheers,
Marlous