Yes, you can safely delete the windows.old files. Windows is kind enough to save them for you in case you weren't smart enough to back up your data when upgrading. In that case, they would still be there for you to rescue.
But in your situation, if there is nothing in there that you need, you can delete them safely. Windows does not use any of those old system files to run. They're just the extra baggage that gets left after an upgrade (like I explained above).
You can even use a program like File Shredder
to completely get rid of them (unnecessary, but possible).
Don't forget to back up your computer before and after you do anything important (do it on a schedule, too)!!