How to change temp directory

Ok, I made the mistake of installing Windows 7 to my smallest hard drive a year ago. It is always running low on space now. I was wondering if there is anyway to redirect all or most of those things that automatically install to the C drive, mainly the program and app data files and temp folder. I know most applications give you the opportunity to choose installation drive, but I also run into the occasional app or installer that doesnt give you that option.

backup all of your files. bite the bullet and re install to the correct drive.

Another approach you could try is to install a bigger drive and use disk imaging software like Acronis True Image to clone your existing Windows hard drive onto the new bigger drive.
That way you dont have to backup anything. Its a seamless migration.