Why is your Windows folder 15 gb? Mine is 8 gb. I have 32 bit but doubt it would be half as big. I'd go through it to see if there is another OS in there (duplicate files).
Short of that, can you clean up some bloatware or old files? Try Disk Cleanup to see if it can compress some for you. Then use the CCleaner "Run Cleaner" and Registry tab.
If you haven't culled the bloatware out of your Programs list, now is the time. Use the Revo Uninstaller to get it cleanest possible and ready for Upgrade without importing Vista corruption.