I’ve upgraded all of the components of my laptop(Dell 700m) one by one, and the only upgrade that made a huge difference for me was upgrading the hard drive from the supplied 40 GB 4200 RPM(Rotations per minute) hard drive to a 100 GB 7200 rpm hard drive.
After the upgrade windows booted up in literally half the time, and programs opened up without any delays(e.g. firefox used to take a good amount of time to open up from when windows started, now it opens immediately!) and the computer in general freezes up less often!
Here are a few great deals on 7200 RPM notebook hard drives by hitachi.
All are from zipzoomfly, (which is where i ordered mine from) and they all include free 2nd day shipping!
60 GB 7200 RPM HD-$120
80 GB 7200 RPM HD-$142
100 GB 7200 RPM HD-$170