You can stop any install at any time...nothing bad will happen, installed files will be removed. There should be a "cancel" option on just about every screen of the install process.
You can then boot into your current install (after changing the boot sequence in the BIOS) and move data files (movies, pics, etc.) to a different partition. My suggestion would be to remove the contents of whatever is in My Documents...to another drive.
You should also dump your Recycle Bin contents and uninstall any programs which you really don't use but which are currently installed.
You should always ensure that there is approximately 15-20% of the partition space available to Windows, classified as unused, free space.
Then...you go back to your repair install