I would organise my files into used-regularly and used-infrequently. Zip the infrequent files. Windows these days runs files from archived formats.
Download Ubuntu-6 or Simply-Mephis-6 (Ubuntu based) for an easy partition and install. Ubuntu is best on existing windows systems but Simply Partiutioner works better.
If you have an older system you would eb better to use Vector Linux. Based on Slackware (the UNIX of Linux) so is VERY stable and it is faster than Ubuntu or Simply. Incidentally it has xfce with ROX (I use KDE, yeah I know bu tit is for work on my laptop and presentations so others find it more familiar).
To not be able to acheive this your hard drive would bneed to be less than 20Gb and since 40Gb has been the standard for years then there should not be a problem. If downloading is a problem, go and buy a linux format mag at £5. This should have at least one distro.