You can make the forms open in a new browser window by doing the following:
- Admin > Service Catalogue Settings
- Browse to the form you want to change
- Click the lightning bolt then 'Edit Form'
- Tick the checkbox 'Open In New Window' and Save.
Note that this will cause issues if you are using the Service Catalogue web-part. It will ask the user to login each time the form is loaded from this web-part. It is an issue logged with Symantec.
If you only use the 'Start New Process' tab then this won't cause any problems.