Yes, it is possible to play game like counter strike, half-life in 64 bit windows. I am playing condition zero on my Windows 7 home premium 64 bit machine.
Please follow below steps to avoid error message like ‘available memory less than 15 mb’ :
1) Right click on Application(.exe) file of your games [like czero.exe]
2) Click ‘Properties’.
3) Go in ‘Compatibility’ tab.
4) Choose Windows XP Service pack 2 as compatibility mode and Apply the setting.
5) Now you are all set to play ~Frags On~