The best way to make it run faster is to format it. But if you're going to end up spamming the hard drive again that's not going to help you for too long. How much hard drive space is left? I never let my hard drive go under 10-20GB, but that also depends on how big your hard drive is I guess.
Try to defrag it first, if that doesnt really help, uninstall any programs you don't need, and set programs that automatically launches on windows start up to off.
But I say if after everything nothing works, format is your only choice.

Hopes this helps.

Thanks for the help, i guess ill delete stuff and get Windows 7.

Start->Run->type "msconfig" -> Go to "Start" tab. If your PC takes too much to start, you'll probably see a lot of ticked boxes (15+)

Now, you'll want to leave it in less than 10 boxes ticked (I have 7 ticked, 24 unticked, and my PC starts in a minute, more or less). How do you know what to untick? Google and common sense. Google each of the "start elements", you'll get a good idea of it's essential or not for start. If it's not essential, it can be unticked safely, but of course, there are non-essential programs that you want to run at starting (like, dunno, MSN may be important for some), that's where it's your choice what to leave ticked and what not

This is actually a "lite" way of improving the startup time, of course there's a much thorough way and it's tampering with services, but that requires a bit more judgment. It's located (on XP) in Control Panel->Administrative Tools->Services, and you can choose to stop services from starting automatically. Most are pretty self-explanatory, so if you don't think you need them you can disable them

HARD MODE: Run regedt32.exe and search for these keys. Most of your startup entries will be in this key:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run

There may also be some stragglers in these:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Runonce
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Run
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Runonce


EASY MODE: get an application to do all this. Something liiiike: http://www.brothersoft.com/windows-startup-editor-11849.html

EDIT: Zex's method is better.

Windows is slow. try Ubuntu ...... actually don't its pretty complex