All I can say guys is that both those games work fine under XP64, I play both so can vouch for this.
However before I played them I ensured that Q4 had the latest patch (quake4_1_0_5_beta2_patch.exe) which I got from www.Gamespot.com.
After I installed halflife 2 I fired up steam and let it do all its updates. Again It runs on my system with no issues. I presume you guys have the latest drivers for such things as your graphics cards and sound card?
AMD Athlon 64 3500+
1GB PC3200 DDR
DFI Lanparty Nforce4 ultra-d MOBO
PNY 6600GT PCIE
SB Audigy 2zs
Akasa EVO33 H/s fan
Hiper Black label 580w PSU.
MS media pro keyboard
Logitech MX1000 laser mouse