Looking at your post I see you have several antivirus programs installed. I suspect they are tripping over one another. Decide which one you will keep AVG,Kerio,.Avast,and uninstall all the others. You should only have one antivirus program on your machine. See if this helps.
My XP home bootup usually takes 15 min (Dell 4600 Pent IV, 512 MB, 55 GB disk, SP2) and sometimes not at all, hanging at the click user screen (which unfortunately means a hard reboot). Strangely enough, if I can successfully logon, everything runs pretty fast, as before!! I should add that my partner cannot log on at all at the moment! A first run of HijackThis eliminated one piece of malware. However the bootup is just as slow. I have used AdAware, Spyware Begone, Search and Destroy, IE-Spyad, CWShredder, MS Antispyware, LSP-Fix, AVG, Prevx, Antivir XP, Norton, Avast, Clamwin, Spyware Guard, Spyware Blaster, as well as ErrorGuard, Registry Mechanic, Registry Medic. HD Tune has showed up no problems (apart from 0.1% bad sectors). I have Spyware Guard, Norton 2005 antivirus and Kerio resident, although Kerio very rarely loads at the moment. Is it forum policy to send HijackThis logfiles? Many thanks!