Can my computer need to be retired after only 2 years?

Here's problem - maybe someone here has ideas:

It has always been slow to boot up - several minutes. However, in recent days, it is taking 45 minutes to boot up! The little dots under the Windows XP logo start moving across the rectangle they are in, but they stop after a few seconds, and just sit there 45 mins.
Here's what I have done:
Scanned for viruses - None
Was on phone with HP and closed all applications at Startup excpet 2 essential ones - No help.
Reset the computer back to a Year ago's setting - No Help.
Defragged drive - No help
Emptied Recycle Bin, compressed files, removed many items from desktop, used Repair tool for hard drive, Deleted cookies, Temp files, browsing history - a few other things that I no longer remember.
All to no avail.
Tried rebooting in Safe mode - still takes all that time.

Frustrating.

Anyone have anything else they think I should try?

i was thinking of switching to a Mac, but not quite yet.

THANKS