Need help upgrading my computer please.

My computer has been going very slow the past month or so and i dont know what the problem is. When im in WOW and i try to Alt+Tab it is very laggy and it locks up for 5 minutes until i can actually do anything. Also its just slow going when trying to run multiple programs.
MY question is what should i do? Should i just reformat my computer? or should i upgrade my computer? Thanks for any advice.

I built my computer about 3 years ago the specs are:

Windows XP Pro

2.10 gigahertz AMD Athlon XP
128 kilobyte primary memory cache
256 kilobyte secondary memory cache

80.05 Gigabytes Usable Hard Drive Capacity
7.75 Gigabytes Hard Drive Free Space

Bus Clock: 166 megahertz

Slot 'A0' has 512 MB
Slot 'A1' has 512 MB
Slot 'A2' is Empty

RADEON 9600 SERIES [Display adapter]
RADEON 9600 SERIES - Secondary [Display adapter]
Mitsubishi DPoint SB70 [Monitor] (16.1"vis, s/n 3570912YA, May 2003)



Thanks again!!

How many other programs do you have running in the background?

I have World of warcraft and i have Ventrilo and that is it.

I think i know what is causing the lag tho. Its all the mods i run in WoW

That could do it.

What mobo are you running and what speed is your memory rated at?

Also, my dada had a similar problem with slowness without any reason and it turned out to the HDD which was on the brink of death but wasn't giving out any warning.

My mobo is a Epox 8rDA+ and my memory is 512 MB 333MHz DDR PC27000 DIMM