Click Start > Run
Type 'msconfig' (without the quote marks)
Click OK
Select the Startup tab
Take a look at the programs which are in the list to run as soon as you start your computer. (You might have google some of the obscure names, or ask here, to work out what they all are).
Click to remove the ticks alongside anything you think might be unnecessary. (You can reverse the procedure later if you change your mind).
Click' Apply'
Click 'OK'
Restart your PC, to see if starts faster. (You'll see a notice telling you that you're using 'Selective startup'. Click to place a tick alongside 'Do not show me this again').
If tidying your startup list doesn't improve things, reinstalling Windows (without uninstalling it first) might help. Exactly how you do that might depend upon which version of Windows you've got. With XP you simply put your Windows disk into the drive and follow the instructions. You don't lose any programs or files by doing so but you do tidy up the registry, which can make boot-up much faster.
Chris