Use my abovementioned steps to recover your old wallet.dat money, then send them to your new wallet.dat address. Just be sure you do not throw away any wallet.dat which may still have some Bitcoins in.

Wallet.dat basically contains "passwords" to your Bitcoin addresses. In the public log of blocks, there are transactions targeting your address, that's how you got your first BTCs. Your bitcoin client just needs to "rescan" the log to show you the Bitcoins you have at these addresses.

The information that you "own" some BTCs at some addresses is stored in all-people public accessible log of blocks. Your wallet has passwords which can "claim" that you own your addresses. That's why you can get BTC to an address without running bitcoin client at all, for example.

So:

1. keep somewhere your receiving address from current wallet.dat
2. backup current wallet.dat
3. restore old wallet.dat
4. run bitcoin -rescan
5. you will see your "old" BTCs, send them to address from step 1.
6. restore new wallet.dat
7. run bitcoin -rescan
8. you should see new BTCs you got from friend plus a transfer of old BTCs from old wallet
9. use the new wallet.dat only

Let me know if this worked for you ...