Windows 7: Ram usage while installing games and pc freeze

I've had this problem only on windows7, for every game I've installed.

Basically, during the installation all of the RAM is used up, and when it ends the system won't free up the RAM, the PC is dead slow and I have to restart it.

It's not a hardware problem, these are my specs:
Intel Core 2 Duo E8400 @ 3.0ghz
RAM DDRII 4GB 800mhz
Motherboard: ASUS P5Qpro
ATI RADEON 4850 Toxic 512mb
Windows 7 RTM x64

has this happened to anyone else?
it's never happened to me with Windows XP, nor Vista, or even Ubuntu.

i used to have 2GB RAM, then when this happened I upgraded to 4 but it made no difference.

I've noticed that my RAM is almost up to 100% when installing a game, but after it's done, it went back to normal.

Ok few quick question.
1) What other task are running when you are installing a game.
2) What Antivirus/firewall are you using
3) Make sure you have all drivers updated
4) run a defrag before you try to install a game
5) Are you overclocking?

ok let's see:
1) and 2) I'm running Windows7, active programs are AVG Antivirus (full version), ASUS EPU6 Engine (monitors and varies CPU speed according to usage), and not much else really... except for drivers (ATI Catalyst software, bluetooth drivers, etc.).
I'm using windows firewall.
It's exactly the same configuration I had with winXP and I never had this problem.
3) drivers seem to be updated, surely the motherboard and intel ones are...
4) good idea, i'll try that next time
5) no overclocking

it looks like it doesn't automatically free RAM. If I check the Task Manager performance, it'll tell me that I have 98% used ram, but in the Processes and Services tab it won't show what program is using it.... it's not ACTIVELY being used, it simply doesn't free up after installing the game.

hhmm I would try to disable your antivirus and install a game. also try to disable ASUS EPU6 Engine too.
I would also check and see if there is a newer copy of the ASUS EPU6 Engine.

thanks, I'll try as you said and then report back