On 1.5 it's kinda easy:
1. Click start, then click run. In the run box type regedit then hit ok.
2. Click the + next to HKEY_CURRENT_USER
3. Click the + next to Software
4. Click the + next to Valve
5. Click the + next to CounterStrike
6. Click on Settings
7. On the right, double click where it says key.
8. A box should pop up with your current CD Key. Change it to what you want to use.