Slow start-up could be caused by a number of things.
Assuming that you are running Windows, here are four that you could try:
1. www.spybot.info - if your computer is infected with spy-ware, this will help you get rid of most of it, if not all.
2. www.microsoft.com/technet/sysinternals/utilities/autoruns.mspx and download autoruns. When run, it will display a list of programs that are run when your computer starts up. Be careful using this, there are some programs/processes that have to be there.
3. www.ccleaner.com - this program will clean up all the temporary files on your hard drive - whilst they may not be a huge contribution to the slow start up, it's good to get rid of them.
4. Make sure that your virus checker is up to date, then run a full scan. If you have more than one virus checker or more than one firewall (e.g. ZoneAlarm), try to rationalise them down to one for each...
Of course, the best way to return to good performance is to re-format and re-install the operating system!
HTH, let us know how you get on.