Thats actually normal of one of the Front Side buses, I'm sure when not overclocked.
It must be something to do with your windows installation, and in which case, i'd actually backup, do a format of your hard drive and re-install windows. You'll definetly get an improvment doing that if so.
You could also try defragging your hard drive from safe mode within Programs, Accessories, System Tools, and this might help speed your computer up by re-arranging files the computer needs to run like startup, and so this might give you a boost.
It seems you've had the installation for a long time though, and is it just the bootup thats slow? If so, defragging might help a little bit.
It could also be a bit of driver interference maybe conflicting, causing the comptuer to start up slowly, when loading
I hope this helps