slowing and frequent freezing

PC has suddenly become slow and freezes frequently, cause is unknown.

On freeze: Mouse is responsive yet programs are not, ctrl+alt+del will not bring up close program dialogue.

Problem began about 3 days ago for some reason, I have run ScanDisk, DiskDefragmenter, and a Norton Anti-Virus check to no avail.

More Info: Win98, 256MB RAM, 600E MHZ, Intel III processor.

My once agreeable PC has now become a slow unstable bundle of chips, I need help! Please.