64bit os should be able to use 4GB ram fully but there is sometimes hardware reserved memory for some reason lige VGA etc.. you should check your bios also check you motherboard manual for ram support.
100% cpu usage all the time may mean that your system is infected with virus or there some software that you dont see running in background. do a clean start through msconfig.
best solution would be buy cheap celeron G550 and H61 motherboard + ddr3 ram as needed
pentium d is old CPU from netburst architecture that failed. G550 celeron in most cases would be sicgnificant upgrade.
Hello, and welcome to Seven Forums. I noticed that you have yet to fill out your system specs on your profile. Doing so will help us assist you with any future problems you may have.
Official Intel specs for that CPU say it had 64-bit instruction set, but only 32-bit physical address extension. I am pretty sure that is the RAM issue.
EDIT: Some more Google searching... Also have you tried making sure PAE is enabled in your BIOS. That might help.
As for CPU usage, have you checked to see which processes are using the most CPU in Task Manager?
Also, it looks like you may be able to upgrade to a Core 2 LGA775, which would likely be a significant improvement over a Pentium D, especially if your PC can use the quad.
My Computer
Computer Manufacturer/Model Number
iBuyPower Chimera X58
OS
Windows 8 Pro x64
CPU
Intel Core i7 920
Motherboard
Asus P6T Deluxe V2
Memory
12GB (2GB x6) Triple Channel DDR3 1333 MHz
Graphics Card(s)
ATI Radeon HD 4870 X2 2GB GDDR5
Sound Card
Integrated Digital HD 7.1 Surround Sound
Monitor(s) Displays
24.6-inch ASUS VW246H 20000:1 Contrast, 2 ms response time,