My works computer only has a 20gb hard drive, 18.7gb useable. My problem is that windows keeps swelling and is 7.4gb at the moment, can I delete without losing any functionality? I am guessing that it is storing updates, so can all but the latest be deleted?

Yeah, just right click on the C drive and press format. That will give you a nice clean 20gb to enjoy.

I think windows has all but killed my current home computer, seems to spend most of its time crunching away on the hard drive and just booting up takes simply ages. It use to be quite a fast machine and every time it does an down load of the web for so called upgrades it seems to get even worse

My home PC had got to be very slow and unreliable.
After trying all the usual tidying activities etc, I bit the bullet and did a reformat and clean XP installation and then downloaded & installed all the service packs and updates and it's now booting up in 40 seconds or so.
I don't know why I put up with slow performance for so long, I should have done it sooner.

I generally reinstall windows every year as it makes pc so much faster. I am going to do it soon to this one as its been nearly a year now

Reinstalling is not an option as it is part of a Novelle network and we don't have the installers for much of the software

Check in C:Windows (ensuring you have set Explorer to show hidden files) and you should find a lot of folders with names like '$NtUninstallKBxxxxxx$' were the x's are usually a 6 digit number. These folders contain the uninstall information for all the security patches and other MS updates, and can be safely deleted if you have seen no problems. Obviously this means that you won't be able to uninstall any patches for which you have deleted the uninstall files.

The other favourite is the temp folder:

Cocuments and Settings<username>Local SettingsTemp

This gets chock full of crap after a while. Perform a reboot, and before launching any applications go into this folder and delete everything.

[Edited on 12/2/09 by MikeRJ]

Theres a handy utility you can download called ccleaner (Crap cleaner http://www.ccleaner.com/) that does a good job of getting rid of hidden stuff you dont need anymore, I use it wuite a lot at work to free up c drive space.
You can do a scan with it first and then it will tell you how much space can be recovered and what it will delete. You can also specify what things it will look for, such as old updates, internet cache files, etc.

I use O&O defrag which runs all of the time and keeps the disk from fragmenting,
COMODO registry cleaner keeps links in the registry clean and working
and Tune Up Utilities to speed up Windows and you can use this to reduce the number of backup points for windows as well.
Finally I use Avast for virus protection.

[Edited on 19/2/09 by Schrodinger]