http://www.filehippo.com/download_ccleaner/
Superb little programme, ive used it for years, one thing to take note of though, if you just install it and run it, it will delete alot of shit but one of those things is cookies which will mean it will clear ALL your saved passwords/usernames for websites, so make sure you either remember them all OR, go into the settings and UNTICK cookies and then it wont touch any of that.
But anyway, run that and tell us how much space it frees up
Other thing to do, go into control panel then "add remove programmes", from here just uninstall anything you do use/need.
Lastly assuming your using win xp, click start bar, then "run" and type in "msconfig" and then click on startup tab, and these things are programmes which load up automatically with windows when you switch the pc on, so have a look and untick anything you dont need as soon as the pc starts up, for example itunes etc. Some are obvious i.e msn, yahoo, but if theres something which you have no idea just google it and it will tell you.
Do all of that and you should find its abit better, if its still slow might want to look into your antivirus and checking you dont have any nasties in there.