Im 24, just over half your audience! Some really sound advice here.
A few things to remember though.
The Windows installer will move your previous install into a directory at the root of the disk (Windows.old)
So trying to remember installed programs can be found in the program files & program files (x86) If your going from 64 bit to either 64 bit or 32 bit. Just remeber to reinstall them, They most probably will not work. Just search for the title in your favourite search engine to find what came from where! Your personal files will be in Windows.old/users (Vista) or windows.old/documents and settings (XP and 2000)

Also, there is a free alternative to Acronis called DriveimageXML, It is as easy to use and as quick, I used it recently when a harddisk was sounding its last moments (click of doom)
I have used acronis before to backup my fathers machine and it was suprisingly quick. (it was a 400 gig)
I used DriveimageXML to back up the adforementioned 500 Gig drive (Maxtor, may I add) and was done in a comparable amount of time (two cups of tea!)
Without further ado http://www.runtime.org/driveimage-xml.htm
I hope you find this useful
Its well written and is very stable!
Regards, Mark