I run 2 Dell computers, a P1 with 32 Mb RAM and a PIII with 128MB RAM. Both are running W98. I find after about 12 months they start to lock up with increasing frequency so I format the hard drive and re install windows and programs I use frequently.
Some may find this a drastic step but I have found it a very effective way of speeding up my machine. As I run a network it is easy to save the data to the other machine whilst I reinstall the software but a CD writer could perform a similar function.