Normally Sage 50 remembers that last check format you used and selects it automatically. But since you just loaded the software onto a new computer it is probably just taking the first check format in the list. When printing checks (or any other form) there's a Select Form button in the print window that let's you choose from all of the available formats.

If this is a single user installation, it's also possible that your custom forms didn't get copied to the new computer. The easiest way to get them back is to restore a backup that was made from the old computer. When you get to the Restore Options window, check the box by Customized Forms and un-check all the other options. As long as you don't have the Company Data box checked your current data won't be affected. But if you leave that box checked, your current data will be replaced by the old data from the backup file so be certain you have the right boxes checked.