Can i install windows 8.1 OVER ubuntu?

You would boot from the "Windows Installation Media"
Then when asked where to install (HDD Partition Screen)
Delete EVERY partition
and point to the unallocated disk space
Windows will create it's own partitions....

That is if you don't want Ubuntu and want to get rid of it. If you want to dual-boot, you must use a utility in Ubuntu such as Gparted to shrink your Ubuntu partition to make some space (at least 30GB) and then create a new FAT32 partition (or directly NTFS if possible). When in Windows Setup, you select this partition, you format as NTFS and proceed. You should be able to boot into Windows but not Ubuntu. Use EasyBCD to add Ubuntu at Windows Boot Loader, or use the Ubuntu CD/USB to reinstall Grub so you choose from there.

I love the installer that comes with Windows 8.1. It will set itself up on any HD, that's running without any problems.
It will stop at the appropriate place and ask you what you want to do. When replacing an existing OS, it's advisable to just tell the installer that you want to Delete all the existing partitions, then tell it to use the maximum space available for the new OS. It will pretty much take it from there and pretty quick you'll have a Clean Install.
It's important, that you understand what you want to do, before you start. Make a plan, and then work the plan.

Cheers mates,
TM

Well, that was ten days ago! Did that Win-8.1 ever get installed?
Curious minds want to know.
TM

Gee, that was last year, and we've still not had a report back, whether Windows 8 ever got installed or not.

It probably was. I would wipe Ubuntu and clean install Windows. Then I would install Ubuntu on a virtual machine, just in case I want to play with it. Yes, "play" is the correct term, Ubuntu is not for replacing my desktop since I hate being restricted in both hardware and software. I hate the very steeper learning curve which only makes sense in the minds of Unix gurus. Any normal people is hard to make sense from Linux's logic. in 2023 you still have to resort to command prompt for even the simplest configuration or application installation. This is not the '80s anymore, even then some of us had computers with GUI such as Amiga.

A nice thing about the Windows Installer is that it can install Windows XX on just about any hard drive with an existing OS on it. The installer will stop, and show me the partition configuration on the drive and basically ask me what I want it to do. I just instruct it to DELETE all partitions, and then format the drive however it wants to. It will usually then create at least two partitions and format them for its own use.
And the installation then continues. It makes no difference if the drive was previously set up for Unix, Linux, Win 95 or whatever. It's all GONE! Ya gotta love it!
TM

To dual boot Windows 8.1 and Ubuntu you have to shrink the partition of one to make room. Then use the unallocated space to install the other. If you have Windows 8.1 right-click on My Computer and select Manage to open Computer Management. Then go at Disk Manager section. Right-click on the Windows partition and select to shrink it. Make at least 20-30GB room. Install Ubuntu. You should boot to Ubuntu by default, but also given the choice to boot into Windows. If you prefer to see the Windows bootloader instead, use your Windows 8.1 Setup DVD or USB. At first screen select your language and keyboard layout and click next. At the next screen click on Repair. This should change the bootable section of the disk to load Windows bootloader. At this point you can boot into Windows 8.1 but not Ubuntu. Now use a tool such as EasyBCD to add Ubuntu boot entry. That's it.