Re-install Win or wipe and start clean

3 month old 8400 w/MCE.
I know the best thing to do is wipe (format) and re-install everything, but
I have a lot of crap all ready installed, and only had my 8400 for a few
months, but I need to get some things cleaned up. Specifically any video
editing software I didn't purchase, trials, of adobe, pinnacle, etc, I only
have Roxio 7 (not the best, but..) and what came with dell, Movie Maker and
the sonic lite crap. So my registry needs to be cleaned up, codec's fixed
up, dll's, etc.
So all in all I know the best performance would be a format and install XP
MCE from the DVD, but can I install over the existing copy and achieve most
of this? I have so many little apps I'm thinking it'll be a 12-16 hr job to
format, install, update everything.
The main reason I want to go clean is so I can get the DVR function (MCE) to
stop failing over 50% of the time! The fall season is getting here fast, and
I desperately need a 3rd recording device (All ready have 2 Tivo's) to catch
all the shows I want while I'm at work during prime time.
Can MCE even be installed over itself (repair I suppose it'd be)? If I
format, should I keep the 4M and 4G partitions (drive F & G)? I suppose one
is the dell Image, even tho I have no option of creating a restore CD.
Should I try some reg cleaners and live with it until I'm ready to do a full
wipe and re-install, which I seem to do about yearly.

TIA
Bryan