Hello

I've bought mikrotik on CF card long time ago, since now I don't need it, I want to use it on xen, and here are few questions - as the license is connected with CF Card ( I think so ), so if I format it and install on it Mt using, will I have the license active ? I hope i've explained it well

If You still have the Card with RouterOS loaded, try to plug it into the computer and use it as a physical disc.
maybe works...

If you have RouterOS installed on the CF, you can use it as disk, if there is RouterOS installed you can try to reinstall new verison of RouterOS using netinstall, to make sure license does not change and device is formated.

NOTE: if external tools are used for format you will loose the license. Use CD-install or netinstall.