Good idea, but I already did that. Probably time to upgrade.
Its likely more a sign of needing to reinstall windows and format the HDD to clean out all the unwanted TSRs than it is to get a new PC. I checked 5 minutes boot time on mine, but my PC is loaded with TSRs. My virus scanner is a real resource hog, and my network stuff slows the boot down too. But it isnt that my pc is old (3.06G p4, 2 Gigs of ram, Radeon X850 XTPE Video, ATA133 80GB HDD). My HDD is defragged too, its just that i boot a lot of crap all at once to start.
Just think back to how fast it booted when you first bought it/built it. if it was fast back then, then you know its not your hardware =p. PC equipment doesnt get slower with age. it either works fully or it doesnt.