My Partitions are overwritten , How to recover previous data?

This is my story.

I have an old CPU 20Gb IDE hard disk with 4 partitions. Yesterday I switched on that machine and I couldn't boot the system. Error " invalid system disk"

Then I opened the case took out the hard disk and Installed it on my currently working machine.
Then I looked for the partitions of 20Gb hard disk under windows vista My computer.
Partitions were not there.
Then I checked using disk manager in admin tools. In that the hard disk was installed but the partitions were not there. And the disk status uninitialized. Somehow windows vista disk manager said I need to initialize disk before assessing it and i clicked OK and went through a process of creating a New single partition in the 20GB drive. Now I have a drive with a 20GB single empty partition.
But I need my data back which were on my previous 4 partitions. I didn't delete those. VISTA overwrite a new partition on previous four partitions. I I think my boot sector is also damaged.
How can I do that?
Please help.