Slow system

I was told that I should keep my computer running all the time (or at least
that linux was designed/could do that; so yesterday I turned it on and went
to school for the day. When I got home the machine was still going (which
was a good sign) but when I loaded a program it took three times as long.
(for instance: I loaded kspread <which takes 10 seconds normally> and it took
like 30)

Any idea as to what the problem is?
--
<:-)~MIKE~(-:>