My computer asks me to choose which OS I want when I boot
Judging by the tag, the OS that you left on your HDD is Windows 7. In that case:
Ensure that any boot partitions from the other OS are truly deleted. You can do this in Disk Management.
Open a command prompt and enter the command
bootrec /fixmbr, then restart your computer.If you still have the same problem, enter this command as well:
bootrec /rebuildbcd
Ooh, fancy, what do those commands do?
Depending on what other OS you installed, it probably created it's own boot partition with it's own boot manager, and disabled/deleted the boot records that Windows set up. You deleted the partitions for the other OS, but left the boot manager intact.
The above commands repair the Windows boot record (MBR) and rebuild it, respectively.
Windows keeps a list of available operating systems in a file on the hard drive which will not be affected by deleting the partition. You need to access this list and remove the operating system you don't want to see.
Open a run dialogue box (Windows key + R) and type 'msconfig' which will open up a window with several tabs. Click the 'boot' tab and you will see the list of operating systems that are offered. Select the one you don't want to see in the list and click 'delete'.