'System' process uses 5% of CPU on battery

Hello,
I have problem with my Windows 7. When I am on AC, everything seems to be fine, but as soon as I disconnect my PSU, CPU ussage of process called 'System' rises to 5% in idle and stays that way until I plug in cord again. It isn't related to CPU power saving features, as forced it to stay at lowest mode (SLFM) with throttlestop in both AC and DC mode. 5% doesn't seem to be much, but I believe that it prevents CPU from going to deeper sleep states.

I have Windows XP too, and there isn't such a problem. When system is idle, Task Manager reports 0% cpu ussage. Discharge rate is also lower (10,7W vs 11,7-12W when idle).

I don't know how to analyze discharge rate figures. I had nc4200 previously, which was designed to run Windows XP, but under W7 discharge rate was lower (by 1-1,5W), and battery lasted longer (~20min). Now i don't know if W7 bigger battery drain is related to problem I described above, or it's just how it is with my current 6910p. I think it should last longer with Windows 7, as it was designed to run Vista, and is officially supported by HP to run 7. Maybe there is another 6910p owner which can provide me with information about battery usage in XP and Vista/7. That would be great!

I have another question about ATI Catalyst Control Center. I occasionally play games, but with X2300 I usually need to reduce resolution in order to get decent framerates, but driver doesn't stretch content to cover whole screen. In order to do it, I have to reduce resolution in desktop to something lower than native, then revert to basic view in CCC, and there is resizing option. As soon as resolution hits native 1440x900 somewhere (in desktop, or game), driver 'forgets' that it was supposed to stretch image and I have to repeat whole procedure again. Under Windows XP i have this option under Notebook Panel Properties > Attributes in advanced view and it works flawlessly. Under Windows 7 this section is missing (as well as few another things), and it is very annoying. Does somebody know how to force driver, to always stretch image?

With both - Windows XP and Windows 7 - I am using newest drivers downloaded from HP site. Both systems are clean: I am concerned about system perfomance, and I don't install any unnecessary software.

Here is my 6910p config:
T7700 Dual IDA enabled
2048+1024MB RAM (one ori 1024MB stick replaced with 2048MB)
ATi Mobility Radeon X2300 64MB
1440x900 screen
Intel 4965AGN Wireless
Bluetooth (disabled in BIOS)
No WWAN chip

Thank you very much in advance for any help.