If this is a name brand system:
Make
Model
Operating System
Age of the computer
Memory Total
Hard Drive Size, How much free space>>>>>>>>>this is very important that I get this info
If this is custom then We will need the following:
The CPU brand and model
The Motherboard brand and model
Ram amount and speed
Cd\Dvd rom interface IDE/
Hard Drive size and free space
Any cards in PCI slots
Any software installed\updated and driver updates done just before problem started