There's a lot of ways to free disk space and move / backup files. Here's a few to get you started, you can do either one, a few or all of them if you wish, but if unsure it's best to leave it.
Recycle Bin
When you don't want a file anymore you select and 'delete' it right.
On your desktop you should have a 'Recycle Bin', deleted files first end up there (to recover if your made a mistake) - this is a space hog.
If your sure you do want any of the files there, make sure it's empty to free up disk space:
Right-clicking 'Recycle Bin' icon and select 'Empty Recycle Bin'
Under right-clicking there's also 'Properties', there you can set the 'Size to be used:' to around 200MB (whatever you want) or if keen and know when you delete something you want it gone disable the recycle bin all together. This will cut down on space used when recycling files.
Web Browser Cache Size
On Internet Explorer or any other web-browser you use find under the Tools > Internet Options > 'Delete Browser History' frees up space by clearing out the cached webpages, etc.
Under that settings you can also set the 'Disk Space to be used:' to around 50MB (whatever you want). This will cut down on space used when caching websites (note caching makes them load faster the next time if you visit the same site again and again, but good to clean up now and then).
Disk Cleanup Tool (Quicker basic option which also cleans the ones above)
Start > Run > type 'cleanmgr' (without the quotes) or Start > All Programs > Accessories > System Tools > Disk CleanUp.
Select 'Drive C:\' and tick what you want to clean up.
System Restore Space
Windows Vista reserves a whopping 15% of your Hard Drives for System Restore (or 30% of free space, whichever is smaller), that's can be almost 75GB out of a 500GB HD! This space is not reserved, but will be taken in 300MB chunks each time a restore point is created.
Under Start > Control Panel > 'Backup and Restore' > click 'create a restore point or change settings.'
Under the 'available disks' heading, uncheck all entries. Confirm and hit 'ok'. This removes previous restore points but frees a lot of room if you have a lot of them. Do this only if your computer is running fine and you won't need them.
If you do not want to disable system restore entirely, you have a couple of options. First, you can clean out all previous restore points, recouping disk space both by deleting existing restore files and causing system restore to recalculate its usage based on the current fullness of your hard drive. To do this, follow the instructions above to disable system restore, then click the 'apply' button instead of the 'ok'. Immediately recheck any entries you removed, then click 'ok.' You have now recreated Vista's system restore settings, starting out with a clean slate. The second option would be to manually alter the amount of drive space system restore can eat up (unforunity unlike WinXP, Vista hides this option under 'vssadmin list shadowstorage' (let me know if you want and I'll tell you how, but it's more advance).
Moving files from HD to HD
If you go Start > My Computer.
Your have hard drives listed, C:\ and your external hard drive if connected, etc. You don't want to move the Operating System or any system files onto an external HD. Instead find your documents, music, videos, downloads, application installers, etc (storage files), on your C:\, use CTRL+Click to select multiple files at a time and right-click the folder(s) or file(s), select 'Cut' (or use quick key CTRL+X). Open your External Hard Drive and right-click, select 'Paste' (or use quick key CTRL+V).
Hope that will help you out. Any questions feel free to ask.
