Bozz is right.
I have 2 HDD's in my system (XP1700, nForce2 Ultra, 1gb ram, etc etc)
1. IBM, 40gb, 7200rpm, ATA100, 2mb Cache, 2.5yrs old
2. WD, 120gb, 7200rpm, ATA100, 8mb Cache 0.5yrs old
The WD abbbbsolutely sh1ts all over the IBM in speed even though they are very -similar- in specs. Being a newer drive with better technology even at a very similar spec it is much quicker in everything that it does.
Everything you do in Windows involves disk access so improving the speed and quality of your hdd will be a huge improvement, although obviously not as much as a brand new system (but $140 against $1,000 is a nice upgraded and also if you upgrade later on you can just throw the 80gb WD into the new system and it will battle on).