Minimise windows HDD / process usage

Hi there,

Having recently switched to Win XP and having a look
around on the windows file system, I was surprised to find
a lot of disc space wasted by installer programs. For
example, I installed Adobe Acrobat Reader 6 and NAV off CD
archives, but yet there are installers for these hidden
away in some cache directory. What are these installer
files? (Forgotten their file extension - was it .ins?)
These files are full installations of the originals...
many MB's of wasted HDD (considering I have burnt them to
CD). What else is there in the windows file system that
is not required and can be purged?

Also was having a look through the service manager and
wondering which services i really need and which ones can
be set to manual start. Reading the help text for all the
Automatic ones sounds like they're all required.

Interested in any info or pointers to documentation on the
above 2 issues.

Thanks.