Assuming that Ubuntu came with the computer could be erroneous.
You "may" need an Ubuntu disk if the Recovery Partition
1.) Doesn't Exist, or
2.) the recovery process installs Windows.
Either way, I'd download a recent LTS version of Ubuntu
and burn it to CD/DVD/USB
32 bit or 64 bit would have to be chosen.
If you can manage to open a terminal and type these 2 commands:
and reply here with the output of those 2 commands, it could help us help you make a decision on 32 vs 64.