for xp, i would make sure of a couple things:
1) your AV isn't doing a constant background scan of every file you access
2) winXP isn't indexing pointlessly
3) your power management settings aren't set to the hdd to low power every couple seconds.
your drive really shouldn't be clicking all that much though. see if turning off some of your tray apps makes a difference. chances are you've got some program running that isn't letting the hard drive heads just sit like they're supposed to when idle.
also, try running a chkdsk. it'll require a reboot and a long and in-depth scan, but it's worthwhile. IDE-SMART utilities are your best bet for checking drive health.