System Slowdowns and Errant Meeses

Our company has a small network (12 workstations) – the server runs WINNT 4.1. The workstations are all Windows 98/98SE. Recently, several of the workstations have experienced some of the same problems. A few of them perform sluggishly as the day wears on, and need to be rebooted – memory leakage? Also several of the workstations have experienced problems with the mouse tracking erratically – they start out fine, then crap out as the day progresses. Again, rebooting helps. I’ve done scandisks, defrags, cache cleaning and trashcan dumping, adjusted swap files, and replaced meeses (plural mouse..) Any ideas?