I always tell people: buy a UPS battery backup. A $40 one will protect your PC from power surges and brownouts that slowly kill your hardware. Lost a motherboard to a thunderstorm once. Never again.

I'd post this on the Tom's Hardware or AnandTech forums with your full system specs — CPU, RAM, motherboard, the works. People can't really help without knowing what you've got. The more detail you give, the better the answers.

Don't forget about heat. If your computer is shutting down or crashing randomly, open the case and check if the fans are caked with dust. Blow it out with a can of compressed air. Overheating causes a shocking number of 'mystery' problems.

Newegg is your friend for parts. Way cheaper than Best Buy or CompUSA and the customer reviews actually help you avoid junk. Just watch out for shipping on heavy stuff like CRT monitors. Read the reviews before you buy ANYTHING.

I build computers for a living and can tell you that most of these problems come down to three things: not enough RAM, outdated drivers, or malware. Check those first before spending money on new hardware.