Cannot run any anti-malware

Hi from a new member with problems. I have reasonable computer skills.
I seem to have some sort of trojan/virus with various systems.
IE8 keeps being directed to sites that are trying to sell me things. But Chrome seems OK.
All anti-malware I have tried gives me error message: "Windows cannot access the specified device, path or file. You may not have the appropriate permissions to access the item."
My anti-virus (AVG 8.5 Free Edition) will not scan.
I have tried following your removal of malware instructions but the only software that appears to be operating in MGTools. Even then, it only produced the GetUnKey,txt log - attached as MGLogs.zip. This was after leaving it running for 14 hours!
Do I have any hope or will I need to format and reinstall?
System: Windows Pro XP on AMD Athlon 64 X2 Dual 4200+ and 2GB memory.
Fingers crossed. kande

 

P.S. It is 32-bit Windows XP Pro.
All of the problems are the same in Safe Mode.

 

Many thanks TimW for spending the time to reply. I have been away for a while so that's why you haven't heard from me. While I had some time to think about my problem, I decided that the safest thing I could do was format and reinstall as I had a full data backup. I figured this may end up easier and safer in the long term even though it involved all software installation as well as Windows. It has also provided a good cleanout of my system as it has been running quite well for a few years now.
Again, thanks and I hope you don't feel I have wasted your time.
Keith Hutchinson