It could be the damaged OS, malware or such. A reload could nail that one.
An old hard drive with bad spots could slow the boot to a crawl or say this machine has 64MB RAM?
10 minute boot example at http://www.youtube.com/watch?v=s-vhimNyRTA
When turning on my Dell laptop with XP Pro, it takes at least 30min for the screen to display the home page. What could be the cause of this problem?