DO NOT RESTART! You will likely regret it if you do! Repartitioning an NTFS partition takes a lot of time, and once it starts you just need to let it go to completion.
Having said that, depending on what you're doing with that partition, you could end up with trouble afterwards. If you are simply reducing the size of the partition by taking away some of the space at the end, you should be fine; I've done that many times without problems.
However, if you are resizing AND moving the Windows partition, you might be in for trouble. Windows doesn't like that, and often rewards those who attempt it with the inability to boot into Windows, sometimes permanently. I did it once, and recovery meant booting into Ubuntu, accessing the Windows partition to rescue everything of value, then reformatting the partition and getting rid of Windows entirely. (By the way, it worked out: I've never reinstalled Windows on that machine and I've never missed it.)
The safest way to reduce the size of a Windows partition is from within Windows itself. Once you have repartitioned and cleared out the space for a new OS, then is the time to boot into the Ubuntu (or other Linux) LiveDisk and install.