dual boot - How do I install Ubuntu Server 12.04 without losing my Windows 7 files?

I have 3 partitions on my Compaq Presario CQ 57 in which:

• Partition 1 has 55 GB (it has Windows 7).


• Partition 2 has 225 GB with no data stored


• Partition 3 has 18GB free.

So, I want to install Ubuntu Server 12.04 in partition of #3 but I don't know how to make it and I don't want to lose my windows 7 files.

Please help me.