Yeah, sorry but there is something wrong (and no, not because it's a Dell or has XP) with your computer.
I've been working with XP machines and Dell computers here at work for several years and I've rarely seen a pc take that long to boot up. If I were you I'd look into your startup services and check all the programs that are starting up on bootup, that may be the problem, you may have programs starting that you don't need but are slowing down the performance of your computer. Aside from virus scan are you doing scans also for adaware or spyware? That junk can really slow down your computer fast. Either that or if you still have warranty call up tech support because it shouldn't be taking that long to boot up.