I consider 50 GB for your OS and installed programs partition way too small on a spinning disk. Your OS partition is very lean at the moment so I'll just give the steps to attempt to get you back to where you were before creating the last partition.
You started with 4 not 3 primaries. A, B, C plus system reserved.
(1) All partition operations have risk. You may need to do a clean install or factory restore.
Backup ALL data to an external HDD. Make sure you can do a clean install or a factory restore (ie. you have the appropriate disks).
Don't proceed with the instructions if you cannot do this.
(2) Delete I: making it unallocated. Can use Disk management.
(3) Extend the volume B: to capture back the unallocated space.
(4) Download Partition Wizard v4.2 to do the dynamic to basic disk conversion. The current
free version of PW 5.2 will not do this. http://cid-24a0e9031e104daf.skydrive...4.2%20free.zip
(5) Start PW 4.2. Select the disk.
Go Disk > Convert Dynamic to Basic Disk
then click the Apply button.
Your disk should now be back to a basic 4
primary partition disk. If any of the partitions are not primary PW4.2 can convert them.
If you want more partitions in the future then B: will need to be made a logical partition.
Finally, A:, and B: are bad letters to pick since they are normally reserved for floppy drives.
It would be better to reassign the letters later on.