Hmm, first of all, a lot of people get confused between Half-Life, Half-Life: Uplink and Half-Life: Day One. What do you have? The full game (Half-Life), the two-level 48+MB demo (Uplink) or the initial five to six levels of the game (Day One)? If its indeed the two-level demo of Half-Life: Uplink you are talking about, then do this:
Right click the Half-Life shortcut, select Properties and then under 'Target' change
C:\Sierra\Half-LifeUplink\hldemo.exe
to
C:\Sierra\Half-LifeUplink\hldemo.exe -dev -console
and then launch the game. You should notice a 'Console' button on the top left corner when you load the game, click it to enter the developer console. Enter
map hldemo1
to start the game with cheats enabled, and during gameplay, press ~ to bring down the console and enter:
/god - Invulnerability
/impulse 101 - All items and health
If its not Uplink and if its Day One or the full retail version, follow all steps except the 'map' command. Just click 'New Game', select your difficulty mode and then enter the cheats like before. For a full list of cheats, check here:
*www.gamefaqs.com/computer/doswin/code/43362.html