I just bought a laptop with some pre-setup partitions like recovery partition, OEM partition, a smaller Windows-partition and the Windows system partition. Their structure and size looks like this: 
I don't really like this 650 GB partition for the system… and thought about resizing this partition for a max. 60 GB partition (that should be enough for the Windows-partition), and leaving the rest for my own other data (music, films, etc.) with a program like GParted or anything similar… I don't really want to remove factory partitions, rebuild the whole partition system and reinstall Windows.
Some say it's not a really good practice to resize or restructure any partition after an OS is installed onto it.
Let's assume resizing happens without a problem (I think it should with a program like GParted), and the OS is running.
Is there any problem with that approach? Can it cause any kind of problems?
There may be things I didn't think of.
Best Answer
You have risks so you will want to make sure you know what you're doing. However, I do this all the time. I don't care about the "hidden restore" partition and I do not like having the entire drive as one partition. I'll download Ubuntu LiveCD www.ubuntu.com and use the preloaded GParted application to resize.
I've never seen an OS require Reactivation after partitioning. Adding/Removing Hardware, yes, but not partitioning.
The only thing that you may see is that the system will do a checkdisk but that is it. If you have any Bad Sectors on the drive, you may run into more complications. Always make sure your disk is health (CrystalDiskInfo) prior to doing something risky. Especially if you do not have anything important backed up.