Even though you already have two partitions, you should make two more so you will end up with:
Partition1: Windows operating system-size as needed by Win
Partition2: Linux operating system-Minimum about 3Gig, better to be 4 or 5G. Format 'Linux ext3' (not ext2)
Partition3: Linux swap file (not absolutely necessary but will make Linux run better)-about 0.5 to 1 Gig but can be smaller if you are tight for space)
Partition4: Data-it's always a good idea to have your data on a separate partiton from your operation system. Saves your data in case of a crash. Windows consistently breaks this recommendation by defaulting all your data to 'My Documents' folder in the same drive as the O/S. Without this partiton any data which is to be accessible to both Windows & Linux has to be put on the Windows drive as Win cannot see the Linux partition. Format this drive FAT32 type & the data can be shared by both Linux and Windows. Use all the remaining space for this partition.