i have already 200 songs in my iphone which i synced with my old pc....now since i m using it to sync to my new pc i get these notification to replace all my previous songs with new ones.....is there any way to keep my old songs nd sync with newones

An iPhone is designed to be synced with only one computer at a time. If you want to change the computer that it's synced to copy the music etc. into its library and try the steps on this user tip : Syncing to a "New" Computer or replacing a "crashed" Hard Drive

Some of the steps on that page are slightly different now (it was written for a previous version of iTunes) :

5, File > Devices > Transfer Purchases

6, Reset Warnings is done via the bottom of the device's Summary tab in iTunes

7, File > Devices > Back Up (or Back Up Now on the device's Summary tab)

8, File > Devices > Restore From Backup (or Restore Backup on the device's Summary tab)