Slow boot time / can't use system restore

For the past 2 months, my Windows XP has had ridiculously slow boot times. It quickly gets to the Windows logo screen, but then I have to watch the three blue squares run across the screen for 3 minutes before I see the "Welcome" page. Startup and normal functioning of the computer are slightly slower than normal, but nothing extraordinary.

After a week of this, I tried to do a System Restore - which suddenly wouldn't work ("No changes have been made to your computer" ). Within the past few days, I have tried again and successfully restored the system to any checkpoint after July 14. I thought this may be due to the "System Distribution System 3.0" restore points that I saw, but these are present before and after July 14.

Any help you can give me would be greatly appreciated. Below is some information.

System: Windows XP home
SP 3
1.5GHz
1 GB RAM

Security:
-Avast antivirus - Thorough scan shows no infections
-Spyware Blaster
-ZoneAlarm (recently uninstalled with no relief of problem)
-Windows Defender (recently uninstalled with no relief of problem)

Other:
-Installed iTunes and 90-day trial of MS publisher 3-4 wks ago; uninstalled with no relief of problem
-I have disabled all non-MS startup items with no relief of problem
-I am on a network with three other computers (1 XP Home, 2 Vista); none have had similar problems

perhaps download the Ultimate Boot CD and run the diagnostics on it, and get a memory tester and see if the memory took a dump