Goto Start then Run then type: regedit. Once inside it works just like Windows Explorer. Browse to:
[HKEY_CURRENT_USER\Software\Valve\Half-Life\Settings]
Once here find [in the right pane] a field called: key. Double-click it or right-click and goto Modify. Now type in your new CD-Key without the dashes -OR- leave it blank. If you leave it blank the next time Half-Life loads you will be asked to input your CD-Key, do so