Typically anything above 60C is undesirable though most will run lower than
this. 65-70C would be a sensible alarm setting to avoid summertime
false-alarms.
As for changing-out the computer, the first thing to look-at is whether the
supplied copy of XP has 'foistware' that might interfere with your normal
programs. If so you need to uninstall it, or if it's a lot then installing
from fresh might be easier.
You should then create a user-account for the regular user, and log-on to
that account at least once, to establish the profile folder.
You then need to install the programs, in general programs cannot be
successfully copied form one machine to another. Thus you should make sure
you have the originals to hand.
Finally, the question of getting the user's files and setings onto it. If
the previous machine was W2000 or XP, and had broadly-similar programs, then
the quickest way may be to transfer the entire user-profile, lock stock and
barrel. If not, then you might find the Files and Settings Transfer Wizard an
easier route.
To transfer the profile directly, you need to log-on to the old machine as a
user other than the regular one, and copy the profile to the server. (If
using Explorer, make sure it is set to copy hidden files) Then logon to the
new computer, again as a user other thean the regular one, and copy the
profile into the regular user's folder under 'Documents and Settings' Log on
as the regular user, and you should have the settings and files as they were
before.