Well, your kind of hitting yourself over the head with a 667MHz processor and Windows XP. Sure, it meets the minimum requirements, but, the word minimum is important.
All you need to read a book is fingers for brail, but, wouldent you rather see the words with your eyes?
(I've been working on metaphores lately)
Anyways, one question about the specifications you listed:
"5MB internet connection over 10/100 LAN"
There is no such thing as a 5MB Internet Connection. Or were you talking about the download speed, measured over speedtest.net?
And, now for full scaled troubleshooting:
What antivirus, spyware, and adware scanning programs do you have?
The number one cause of sudden system performance decrease is a virus.
Or did this happen over time?
You might consider cleaning out your temporary files, and the system registry using a program called Ccleaner, available for free at
www.ccleaner.com It probably wont help an incredible amount alone though. While you are running that program, go to the Tools panel, and then "Startup"
Disable (but, dont delete) any startup items which you do not use on that computer.
Second, go to the Uninstall area. Remove what you do not use.