Minimum requirements for installation of Ubuntu
2GHz dual Core Processor
4 GB RAM
25 GB of disk space
Bootable Media with a space of 4 ...
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The other way around would be interesting: How can I install Windows on a device with Ubuntu already in it? My experience was that windows just kills everything that is not windows, and the GRUB boot manager won't set up automatically.
For what I know you can still boot up Ubuntu by selecting the partition at the startup of your PC.
Once you are in Ubuntu you can do the next steps here: help.ubuntu.com/community/Boot-Repair