|"I've accidentally re-formatted my old harddisk and reinstalled a new XP in it."|
You should have removed the drive and connected it externally to another system to recover your files. Recovery software can often recover files after a format, however a reinstall is likely to overwrite some of the sectors that have your files. Continued use of the system dimishes the chances of recovery, because more files might be overwritten. The best thing to do is remove the drive and connect it to another system. The following are some pretty good recovery programs:
PC Inspector (freeware)
Also free - Thanks to 'Emma Lee'
Zero assumption Recovery
The demo is limited
It will only recover 'up to' four folders per run
But you can make multiple runs