HELP!!! I have been receiving this error msg at log-on for some time, and can't figure out how to fix the problem- 'svchost.exe - Application Error - The instruction at "0x7c91b1fa" referenced memory at "0x00000010". The memory could not be "written"'.

I believe this is related to the glitch in a Windows XP update from last year. I've come across solutions for similar error msgs, but none match my particular "number"
(I have double-checked to assure that I have the error msg written correctly)  
ANY help would be most appreciated-Thanks

Are you using NOD 32?

From the start menu: Run:services.msc to bring up the services window
Select "Windows Driver Foundation - User Mode Driver Framework" which will bring up the Properties window.

On the General tab, ensure that the Startup Type is set to Disabled

Reboot

This fixed the problem for me