DD's suggestion is the best :). Espically the last one.

I am adding few more

1. before restoring, try defragmenting the hdd. (start/all programs/accessories/system utilities/disk defragment)

2. Be careful if you are using registry mechanics/repairs. Not all the time your registry will be fixed properly.

3. If your computer boots well ( without slowing) , you can use Avira antivir, that is free and scan your system. ( if it takes forever to boot, use avast)

4. If it detects and repairs lots of virus, you may wanna reinstall the os or recover to the original /factory condition. Follow DD's guidelines up to 8.

5. Dont forget to check which processes/programs is eating up your big chunk of  resources. Kill em if needed.

6. I personally dont think this is overheat problem as your system temporarily freezes but doesnot shuts down ( if i understood it correctly).

If this fixes your computer, strictly follow DD's guideline #9.