Another svchost.exe application error

Hi everyone,

Some googling led me here - I felt a little bit of hope to see some discussion on svchost.exe errors - and then a little depressed to see just how many there are.

I have XP Home SP3, and on startup I receive the classic
"The instruction at "0xnumber" referenced memory at "0xnumber". The memory could not be "written"."

Lots of other people's errors appear to be related to Windows Update - but this computer has never had auto-update on, or received any windows updates. However, when I try to update from the microsoft update site, the error appears (if I have previously closed it). I also get errors saying I have no audio device installed when I open WMP, iTunes, etc. and occasionally movie files will have no audio. If I try to restart, the screen hangs at "logging off".

I've done full malware/virus scans and found nothing, and followed the Malware Guide from this forum (however I can not create a system restore point - I get a DEP error). I also can't run a hotfix from support.microsoft.com that seemed to help some others with a similar problem.

I can post HJT logs, eventviewer details, anything, if someone has even a little idea of what might be causing the problem.

Thanks a lot for taking the time to read.