There can be a multitude of problems which can cause what you are describing.
Here's what I'd suggest you could do yourself (some you may already have done) - try them, and check after each one if it makes the problem any less severe:
1) Download CCleaner & do a full system cleanup - this includes Startup Programs, Cleanup and Registry Cleanup
2) Remove the AVG, since you don't sound happy with it anyways. Download and install MS Security Essentials (also a free app) and try it out. It's less intrusive and the default settings don't interfere with normal workings as much as others
3) Run a full scan of your hard disk, to ensure there are no dodgy sectors - this can be done by going into 'Computer', then right-clicking on your main drive (C

, choose 'Properties', then choose 'Tools' tab. Choose 'Error Checking' and tick both boxes, then 'Start'. It will probably give you a message asking you about scheduling on restart, so do this.
4) Next software-related fix is probably taking a copy of your data off, and reinstalling the OS to a factory default state. This might be outside your remit, but it is an option. Do this, then update all drivers, and install your important apps & AV software back on. Load back your data, then see if the problem persists.
After this, it is probably hardware-related, so it will need a PC Engineer to swap in/out parts to see if they can find the problem. Obviously lots of processes can be tested, etc, but I'd suggest if none of the above help in any way, it's a faulty hardware component.
The type of problem you are describing is very difficult to diagnose without having the PC on a bench to test properly, and the above is pretty much all I'd suggest to a normal user.
hth