There are a LOT of factors for bootup times. But the biggest is the hard drive. While memory and the cpu can do GB's per sec a good hard drive can only do 50 to 100 MB per sec. The new SSD drives are up to 450 MB a sec.
So if the system is older and the hard drive is older you may be waiting for the drive. Windows and anything else needs to be read off the drive to run. So your waiting on the drive during bootup.
Shane