start > control panel (under my computer) > add/remove programs, or programs and features
if the list doesn't have "last used on" at the top, then right click where the sort stuff is and pick "more..." from the drop down menu.
find "last used on", and check that. hit OK, then click it on the top of the list to sort it by "last used on" date. you'll be able to right click and remove what you don't ever use, from there.
a huge eater of HDD space is videos, so if you have any that are just laying on your hard drive, you might want to back them up to a dvd or get rid of them, however you want. if it's a laptop, you have few choices to up your space other than buying an external HDD. if you have a desktop, consider buying an internal hard drive (500 GB should be plenty, but 1TB is how much i have personally, and i've yet to fill it upall the way). not only will you get loads more space, you'll learn a thing or two about the insides of your computer, and how to install new parts. always good to know your own stuff.
good luck.
edit: oh, and make sure you empty your recycle bin. right click > empty recycle bin. if it doesn't show on your desktop, right click the desktop, and pick change desktop items (on the left sidebar, i'm using vista. for XP/win7, google how to get to change desktop items), and check it to show the recycling bin. dumping that frees up a lot, if you don't already do that.
