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Does anyone know, besides ad/spy-ware or viruses, why my programs and web-pages open & close so slowly?

I have already run several bspyware, adware, virus, & rootkit scans. These have all checked out clean. I've cleaned out all the cookies and temporary files. I've defragmented. I have an abundance of free disc space and RAM. Using Windows XP pro. Office 2003 pro. I do have several items loading at startup, but that shouldn't effect opening and closing programs or webpages should it? Could it be my anti-spyware &/or firewalls? I haven't tried running anything with these items disabled yet. Should I do that first? Any sincere information will be deeply appreciated. No smarty-pants answers please, only serious replies. THANKS!
My issue isn't necessarily during start up. It is after the start up. Any time during the day. It can be 20 min. to 6 hr. after I start my computer. IE: When I open a Word Document, or an Excel Page, or the "e" to open my home page on the web, things like this. They take quite awhile to open and load. They also take quite awhile to close when I attempt to close them as well. BUT, minimizing these "windows" is very quick as well as restoring them. It is only the initial opening and the closing that seems to be slow. Does the additional info. help any? I hope so.
I have Symanted Anti-virus, Zone Alarm Firewall, Windows Defender (firewall right?), Advanced Windows Care V2 Personal (spyware protection ?firewall?& other options such as spyware removal, registry fix, etc) I will start w/ disabling all but one firewall and test it out from there. I am comfortable with attempting internal adjustments on my computer as well, and will try these suggestions. Thanks!

A couple of things come to mind:

What kind of antivirus are you using? Norton, Panda and McAffe are known for slowing down your system badly. They can cause that behaviour.
Also, make sure you are not running two antiviruses at the same time. That is just a big no no.
Make sure you are not running two firewalls at the same time either.

Second. Do you have Internet Explorer 7 installed? Keep in mind that it might have been forcefully installed via automatic updates. The good people at Micro$oft have decided that everyone likes IE7 and force it into us.
If it is so, uninstall it and see what happens.
I have seen IE7 slow down a system to a crawl. In one particular case, it was so bad that I was forced to re-install windows.

Check also how your system behaves if you disable your anti-spyware protection (which, by the way, you have not told us what it is). Some anti-spyware programs can affect the system performance.

The programs that load at start up also affect your performance. Try disabling them (either one at the time, or all of them and then enabling them one at the time) and see the results. For example, I have seen MSN Messenger and Windows Live Messenger to affect performance a lot.

Finally, it might be hardware related. I've seen some cases when a memory module is causing a system slowdown. Also, I saw a case (with a very old PC though) where the processor was acting funny, and the computer was crawling. In both cases, the problem was cured by removing and re-inserting the memory or processor. If you feel confident enough to play with your computer's inner parts, you can try that as a last resource.

Good luck.