Does a Core 2 Duo make things faster in this specific case?

AVG is scanning the HDD. The only resource it uses a lot, is your HDD.
If you want it to go faster, you need to defrag your drive, and or use less of the drive.
HDD drives are funny. The fuller they get, the slower they get. I try to never have more than 25% of a drive filled. That usually keeps the speed up.

having reread your post it seems to me that you might have programs running in the back ground.

you might want to check your task manager and see whats running.your rig should not be running so slow.

there are a lot of good online scanners that you could use.

click on the green button on the left http://www.majorgeeks.com/O&O_Defr [...] d4545.html

heres another one for you to try http://security.symantec.com/sscv6 [...] &venid=sym

both of these are easy and wont do damage to your rig .i would try these be for you look any were else.

i dont think from what you said that its a hard ware problem

the registry is something that should be left alone, as for those incorrect entry's - no need to worry.

a Core2 series cpu regardless will make things a whole lot quicker, but is still somewhat limited by the hdd

as for the resource usage - load of bs, the % means nothing as windows reserves resources and so on - hard to tell what its actually taking and leaving etc ready for apps.