Here are a few thoughts:
1. Do you have a lot of applications starting in your startup folder? If so, do you really need them all? I suggest you run msconfig>startup and deselect any items that you don't really need to be running in the background.
2. Have you got a lot of software loaded on the computer that you don't really need? If so, uninstall anything spurious.
3. When did you last do a bit of housework (for example with CCleaner), and then a defrag? After you've done 2. above, it might be a good idea.
4. What security software are you running? Is it taking a long time to appear in your System Tray?
5. Did this slow startup happen suddenly, or has it been slowing down little by little? If it was a sudden event, run a full security scan and/or use System Restore to roll the computer back to a date before the problem began.