Computer has slowed way down

I have an emachines T6520 computer with Windows XP Media Center edition
Service Pack 3 with 2.40Ghz AMD Athlon 64 processor.128 Kb primary memory
cache 512 Kb secondary memory cache. 896 Megabytes installed memory.
My c drive has 150.17 Gb free.

I'm up-to-date on all of Microsoft hot fixes for Windows.
I use AdAware, Spybot Search and Destroy and Super Anti- Spyware frequently.
I have McAfee Security system and recently ran a complete scan with no
troubles found. Usually when I try to defrag it says not necessary to
defrag, but a couple of days ago I did a defrag and it did not help .
I'm far from being an expert, but from what I've read it seems like I may
have too many programs on Startup and too much stuff running in the back
ground using up memory (or is it system resources?) The big users of memory
(using CAD) a
iexplore.exe 23,088K
msimn.exe 54,900K
msntask.exe 38,544K
Mcshield.exe 101,124K
explorer.exe 15,348K
AAWService.exe 18,480K
sychost.exe 17,240K
There are 56 processes listed with CPU usages varying from about 47% up to
100%

Looking at the startup tab in msconfig I see about 25 items listed. Some I
can recognize as names of programs, but most have names I don't recognize
(such as atiptaxx, PDVDserv,Zhotkey Etc..) I copied several of the unknowns
into Google searches and, mostly I found vague descriptions and comments
such as "Not dangerous, not necessary, but recommend keeping it unless it is
known to be causing problems" I unchecked about 6 of them and don't notice
any difference in performance.

Finally, my question:
Can anyone suggest a sensible way to approach clearing out unnecessary stuff
in my Startup and Ctl/Alt/Del list of running processes? It would seem that
if I deleted them one-at-a-time, it would take forever and would be hard to
know if taking one out had made a difference. And, does it seem like this is
a good approach to speeding my computer up?

Thanks for any advice you can offer.

Fred