Agreed on the full wipe. When you go to reinstall on the disk screen format it there. It will take a while depending on how big and how fast the HDD is but when giving it to someone else that you have no idea always do a full wipe.
As for getting around the current user we can't help with that due to the forum rules.
If it came with Vista though it might not run 10 very well. Probably is some lame Core2Duo mobile or a mobile AMD with probably 2 or 4Gb of ram unless it has been upgraded.