This is usually due to a cause of overhead software or hardware added to the machine...
For instance.. If you have so many programs - more than 20 (actual full programs, at least 30+MBs), the system goes through and verifies every single registry addon and linking to the system. Also, fast find or "microsoft office" from the Office series cd's in the startup will slow the system down once logged in.
You are saying that it is on the initial boot process which would mean you have alot of programs or usb hardware installed on your system.
Turn your system restore off.
There are programs out for free called "RegCleaner"... about 300kb's in size, relatively small. Download any version of one and have it do a full registry clean. It will remove links that go nowhere and are not registered in the system. This will speed up your boot process immensely.
Also, if you have not yet, get Adaware, Spybot S&D or both and run them on your system to fully clean out misc. malicious items that got put into your pc. Reboot after each one of these programs finishes with what it needs to do. Should be fine after all of that.
Turn your system restore back on when all 3 steps are complete.
Good luck.