The advice above are the kinds of things you need to be doing...
De-Fragmenting your Hard Drive(s) is a great idea - people rarely do this and after a long time enough adding/moving/removing data will fragment a file system and slow it down pretty severely. As stated, find Disk Defragmenter in the System Tools folder off the Programs/Accessories menu. If you have difficulty with it reporting that it can't complete the job because of access to the file system(s) interrupting it, let us know and I'll recommend some third party stuff that probably won't struggle.
Clearing out temporary files can help, but normally only if things are really badly clogged up - a handy utility is available from
http://www.diskcleaner.nl/ which will help - if there are things on the list you're not sure about clearing though, un-check them.
MSCONFIG is the tool (as stated, run from the 'run' box on the Start Menu) which allows you to over-ride the registry-configured start-up applications - you'll probably end up setting it to boot your machine in a diagnostic/debug start-up, but there's a check-box on the first start up to tell it not to nag you about that.
If you've got any doubts about what particular items in the list, avoid disabling them - post the name and path here and we'll advise you on whether or not it's safe or brave
Also, updating virus/spyware protection can help if your machine's clogged up with junk. Some of the free packages aren't bad but at the same time don't always catch everything. For simple options try AVG Anti-Virus and Ad-Aware free versions. They're by far the most competent but they will help and they're straighforward. Spyware Doctor is a great little package - you can get a free version of that which will highlight what it finds, but you do have to register (not massively expensive) to remove things. As my main laptop is connected to a corporate domain it runs Symantec AntiVirus corporate edition - that, if updated daily, is probably one of the better packages - it sits quietly doing its thing, offers great anti-virus protection and at the same time pretty decent anti-mal/spyware - it doesn't use masses of system resources either, nor does it have all the nagging that are in its sibling Norton consumer products.
Have you got Windows set to automatically update hardware drivers? If it's pulling in driver updates itself it may have updated one which isn't necessarily the best version - poor chipset drivers or disk controller (eg SATA/RAID) drivers can make the Hardware Abstraction Layer run like a pig (and setting up HAL/enumerating devices is probably the largest part of the 'pre-desktop' boot process) - but then fiddling with things like this can be risky if you're not confident. Do you know the full spec of the machine you're using?
Anyway, have a look at the advice further up the thread and maybe what I've had to say, see if it helps and let us know...
Regards,
Rob.
PS> There is equally a chance that you'll not speed it up drastically, the best thing to do in that case is level it and start with a fresh install - but that might not be something you want/feel confident to do?...