First steps to take: make sure you got lots of memory. Win XP will cry with anything less than 256 MB of RAM. Defragment your hard drive regularly. Keep as little background programs (i.e those in the taskbar) running as possible.
If you've had your hard disk for a few years, perhaps it's experiencing natural degradation. Probably you'd want to replace your hard disk in extreme circumstances.
But more often than not, a memory upgrade does magic.