Just to recap for you:
1) First choice is to use a cleaning program. Like mentioned ealier, one of the best (yes I use it as well) is the CCleaner. It is freely downloadable from their website. The registry cleaner is quite good. One of the main causes of slowness and hdd's filling up is files that are left behind, as well as the cache of web browsers. Every time you visit a new page, the browser downloads a temporary copy of the webpage you have visited, to enable you to view it quicker when you go back to it again, so it is always useful to clean these pages out every once and a while.
Normally the file sizes for these are only a few KB big but it does mount up and if left for a really long time it can take up alot of large space!
Now if you do not want to use a program to do this, you can use the browsers in-built functionality. You can access it usually by going to Tools and then "Clear Private Data" (in firefox) or by going to Tools and then "Clear Browsing History" (in IE 7).
2) The next option is to remove unwanted programs. Now if you need all the programs on your machine then this leads me on to the next step!
3) Buy a new HDD! This option costs you some money, but as mentioned earlier on in this thread you can pick them up fairly cheaply these days. Some of the best websites that I fount you can buy from is:
http://ebuyer.com
http://www.novatech.co.uk (UK Only)
Obviously there are more websites that you can buy products from. The way that I normally judge by how good a website is if that someone I know has had a good experience with them. Another good way of judging a website is by using Google. Simply enter the item you are looking for and then look at the Google shopping pages. They show all the best prices for the item you are after, and then also have revies on the suppliers that the product is coming from. If they have a rating of 4.0/5.0 then they are a good company to go with. Obviously shop around fo the cheapest deals.
If you have no room in your case for a HDD or are unsure what to choose then you can pick an External HDD which tend to be Plug and Play, which basically means you plug them in and away you go!
If you got any more queries, do not be afraid to ask!