There r so many Rescue disks in the market. It depends o ur purpose ! As u asked for "Computer is locked up. can't get it to start", that means is it Login problem/ Password mismatched or not booting properly, however,u u can run run repair with the same OS vendor Setup disk to get out this issue/ boot the OS in Rescue Mode, if it allows. If this will not help u, u can use Rescue disk to run ur PC.....
U can dnld Kaspersky Res Disk: https://support.kaspersky.com/viruses/rescuedisk
or
from Avg: www.avg.com/in-en/avg-rescue-cd
or
from SysResCD: www.sysresccd.org/SystemRescueCd_Homepage
or
from Knoppix: www.knopper.net/knoppix/index-en.html
or
simply use Linux Rescue CD.