Win98 = I never had an install be stable longer than 6 months.
Clean install of XP = a must.
XP = very memory hungry.

Scandisk can sometimes find problems with a drive (do a surface scan, even though that will take a long time), but to be perfectly honest I don't think it's ever been useful for any of MY hard drive failures (and after working with computers since '87 there have been a lot).

Even the much-advertised S.M.A.R.T. feature has NEVER detected a problem, even on drives slowly tearing themselves apart with minor head crashes...