Have you cleaned out all the temp folder recently?
A fat temp file will contribute to slower operation.
C:\Windows\Temp, Temporary Internet files, etc.
Have you edited you Start Up list in msconfig?
I disable all except my firewall and AV.
Too many processes starting up at boot can drastically slow the boot process. Likewise at shutdown.
A good addition to your av/spyware suite would be
Adaware. It finds pests that the others don't.
Best of Luck
Current system - Windows XP, SP1 (soon to install SP2), 512 Meg and using 19 gig out of a 145 gig hard drive. Norton Internet Security and Webroot SpySweeper enabled. I defrag the drive and use the above 2 utilities plus Spybot to keep the nasties down to a minimum.
Nevertheless, the system has slowed down, especially on bootup, shutdown and opening files.
Will registry cleaning noticeably speed things up?
I've tried a registry scan using Registry First Aid but I'm relectant to delete even the "safe to remove" keys - a couple mention Microsoft/Outlook. Yikes!
I've read that you should backup the registry first so you can restore if things go wrong, but I'm not sure exactly how to retrieve and copy the backup.
It seems like a very complicated process and potentially dangerous. I guess it comes down to I don't know which of these products I can trust.