Windows XP's Disk Cleanup utility (Start > All Programs > Accessories >
System Tools > Disk Cleanup) will be helpful. Beyond that, it's up to you.
Every user has their own opinion of what is 'junk' and what is worthwhile.
There is no substitute for knowledge and your own good judgement.
In the days of the Windows 9x operating systems, it wasn't a bad idea to
reformat your your hard disk once every year or so to give your computer a
'fresh start' (if you didn't mind spending a weekend re-installing,
re-updating and re-configuring all your software and hardware). Windows XP
is such a great improvement over its 9x predecessors that you no longer need
to do that. Now, the only good reason for reformatting is if your operating
system is in such bad shapes (due to viruses, spyware or lack of knowledge)
that there is no other way out.
As long as we're on the topic, you might want to consider imaging software
(Norton Ghost is an example; see
http://www.symantec.com/sabu/ghost/ghost_personal/.) This allows you to
create an exact copy of a known good configuration which you can later
return to, if need be, in just minutes.
Tom Swift