Try WinDirStat. It can be down loaded, Google for a download site. Windirstat looks at every file on your hard drive and draws a cute graph showing how much space each file is sucking up. Rolling the mouse over the file symbol in the graph calls up its name in the bottom text window. Gives you a a good feel for where to look for fat to cut.
First place to go is Start -> Settings -> Control Panel -> Add & Remove Programs. That pops up this window.
It lists every program on your machine. Now is the time to zap all those craplets and games your machine came with and you never use. Be careful, this window lets you zap darn near everything on the disc, including parts of Windows itself, your browser , MS Office. It's like a chainsaw, cuts fast and cuts anything. Useful to be sure, but you gotta be careful. The Programs with the MS icon of a Computer Monitor, CD and CD Box you mostly want to keep.
When you click on anything to remove it, it will tell you how often you used it and when it was used last. Things that haven't been used since the bought the machine can probably be spared.