Jesus christ! You never played Half-Life? GO! Buy a copy! NOW NOW NOW!!! The whole Half-Life series sells in box sets for $20 bucks! GO GO GO!!!
Seriously, though. HL1 is an AWSOME game in its own right. Not to mention that HL2 builds off of what happened in HL1, so it's worth you playing through.
And JFTR, Counter-Strike is overrated. Aside from the fact that it is the most heavily hacked/cheated/a$$hole-filled mod/game on the multiplay networks, it just gets an average score for quality.
Anyway...Half-Life (original) is a MUST for any PC gamer. That's kinda like owning an original ATARI and not having Space-Invaders.