Well, a dramatic change like that could be a number of things. First thing to check is a network issue. If your computer is on or has been on a network, then there are cases where the machine will just sit there at boot for a few minutes listening for a network that isn't there until a time out interval, particularily if you are using a netware redirector.
Second, it might be a one-time thing. For example, if you machine just download a patch for windows, then depending on the patch, some of the install happens on reboot. Thre are other cases if you jsut installed other software or with some resident programs liek AV. But if you reboot again and it still happens, this isn't it.
Third and most likely thing though is probably some program or driver configured to start at boot is taking it's sweet time. Anti-virus programs are notorious for this. It's amazing how poorly AV programs are written and their file system compenents run in kernal mode. Some sort of spyware is also high on the list of possibilities. Those little weather/advertisement things that pop toast from the tray have stuff that automatically configures to auto-start or it could be somethign you don't even see like a spambot. If you don't run a spyware detection/removal tool regularily, that's certainly something to check.
Even innocent thigns like the Real Audio player have an auto-start component and if it has problems calling home to check for news/upgrades, etc. or a bug it can hang the whole machine boot depending upon how it registered.
Things to do:
Get rid of auto-start stuff you don't want. Check the start-up folder and also the registry. One of them might be to blame. You may need to edit the registry or use a thrid party tool to do this completly. I hate the fact that windows has multiple ways to configure things to auto-start, but it does.
Run an anti-spyware tool.
Try booting in safe mode and compare the time.
Try booting with your AV uninstalled (not just disabled). The kernal mode compnents of AV programs can really tank a machine's performance....
Try booting with the SP2 firewall turned off and/or with your AV program's firewall turned off...
Suspect your drivers. Did you add hardware recently? Did you install some device, an ipod, a flash card reader? Are there updates for the drivers that came with your machine from it's manufacture you should get?
I think it is unlikly that such a dramatic change is due to file or registry fragmentation or any other "normal" windows function unless perhaps you are completely out of disk space...
