Back up everything to an external drive * (whilst you still can) then buy a larger HDD for your PC. (It's only a couple of connections and a latch.)
In the meantime (right now) press 'start' then 'control panel' then 'Performance Information and tools' ... and click on 'Open Disk Cleanup' to delete redundant files.
After that's done click 'start' then type 'defrag' (no quotes) in the search box and press enter, click 'Defragment Now' and wait while your hard drive is reorganised.
Be prepared for these operations to take a very long time if you've never done them before. (Like overnight maybe.)
If it lets you ... download Piriform's free version of CCleaner and run that through. (It'll do a job on all your gash files, cookies, temp files, history etc.)
All of the above is simply to get enough room to make an emergency system backup (or two if paranoid) then buy a new HDD.
When was the last time you cleared out redundant files and defragged?
I had to restore a Windows 7 Pro system from external drive (and bootable DVD) recently when an HDD went kaput. Piece of p### in the end. Make sure you have your Windows Vista unique serial number recorded somewhere you can read it easily if required. (I have it photographed on my iPad so I can magnify it.)
* Obviously a bigger size. (If doing a complete system backup have a fresh, writeable CD or DVD to hand as well as your external USB hard drive. You'll be prompted when required.)