Half-life is unplayable on XP without tweaking.
1. How to recover from incompatible drivers
Before you install new drivers set a system restore point. Start>All programs>Accessories>system tools>system restore
After your new drivers don't work reset your computer. Press F8 repeatedly as soon as the BIOS screen disappears, and before the Windows XP screen appears. Select safe mode. Use system restore again to undo your mess.
2. Video Drivers
The NVidia drivers that come with XP do not allow you to run Half Life in OpenGL. Update to the newest drivers.
Despite the fact that they are not official drivers, 22.50 was the only set which worked
3. Sound Drivers
Use windows update to update Creative drivers.
4. Fixing screen flicker
Windows XP defaults to 60Hz for games. A fix is available here:
Select "set: every resolution to monitor's maximum supported rate"
5. Fixing lag
If you are having trouble with lag, try disabling the windows XP firewall. Go to control panel>network connections. Select connection, right click, properties, advanced, untick the firewall.
You can improve your mouse smoothness for games.
Change the sample rate to a higher one, eg. 200