Do not convert to dynamic. I'm lazy right now, so I'll just suggest that you google dynamic disk and even check the wiki info that shows up.
Either add another physical disk OR:
This is what I would do:
1. make the HP recovery discs in case you ever want to put things back the way they are,
2. Using Win 7 Backup and Restore, make a system image backup of all partitions on the disk to an external usb drive (this is optional, but would give you a complete restore capability.)
3. make a 100% clean install of Win 7. This way you will have only used two of the max of four primary partitions. Clean Install Windows 7