Computer freezes usualy fall to this category:
Hardware:
Defective or mismatched components in your computer.
Aluminum Migration & ESD:
Premature ageing and Electro Static Discharge can make a computer very intermittent.
Operating System:
Some versions of Windows have bugs that can cause these problems.
Drivers:
Buggy or down-level drivers can cause many problems.
Software:
Some programs have bugs in them that can cause these problems.
Memory Fragmentation:
Do you often see the message, "System is dangerously low in resources" If so you are probably suffering Memory Fragmentation.
CMOS Settings:
Improper CMOS settings can cause many problems. Changing CMOS settings can also be used to help diagnose some hardware problems.
Loading Order:
The order in which the software components of your computer are loaded into memory, at startup each day, can cause a multitude of odd problems.
A Combination of the above:
Defects in any of the above may only show up under specific circumstances.