So I currently have two partitions right now on an old desktop of mine.

Windows Vista (only keeping it because it has some important files).

Linux Mint (After using Linux Mint MATE for a while I found it caused my desktop to be a bit laggy so I'm thinking about switching to Xubuntu).

Now, usually how I changed distros is I go to my Windows Vista partition and then delete the Linux partition and then reinstall a new distro with a live cd.

How would I do this without booting into the Windows Vista partition?