Ok, I need to return to Windows for some months on this computer but am having problem with GRUB.

I currently have Xubuntu Fiesty but will be returning to windows 2000. I start the installation of windows (through bootdisks) and almost complete the installation, but by the end one has to restart the computer. When I do restart I get to GRUB (obviosuly errored as the linux-disk is formatted during install), and can thus not continue my windows-install. Has the GRUB been installed with the BIOS or?

How should I solve this problem?

Grub is usually installed to MBR (Master Boot Record) it should get overwritten during windows installation.
If not you could boot the windows cd and choose shell and try to give it command fixmbr. It should write the ntloader to MBR.
If you still get working grub shell you can load windows from there by something like this:
Hope it helps.

my computer cannot boot by cd (have a cd-writer to read), so I initiate the install through floppy boot-disks. Also, The GRUB shell doesnt load properly... As I have reinstalled xubuntu again, I dont currently know exaclty what the error-message is.. I wanted a solution before I tried to install windows again... but will be try now then, and access this forum through my brothers computer for help...

As suggested, this should be resolved by the Windoze install.

If you can't boot the W2K CD, from a Windoze boot disk enter

To complement syg00's suggestion a Windoze boot disk is downloadable from bootdisk.com. Just use any one of Dos 6 or above.