I was looking in my control panel and noticed the message 8.00 GB (3.25GB usable) Im running windows 8.1 on a slightly old ASUS P5Q-E motherboard, it does me fine. But how can I solve this? 
Some info for you....
OS Name Microsoft Windows 8.1 Pro with Media Center
Version 6.3.9600 Build 9600
Other OS Description Not Available
OS Manufacturer Microsoft Corporation
System Name JIMBO-PC
System Manufacturer System manufacturer
System Model P5Q-E
System Type X86-based PC
System SKU To Be Filled By O.E.M.
Processor Intel(R) Core(TM)2 Quad CPU Q9400 @ 2.66GHz, 2670 Mhz, 4 Core(s), 4 Logical Processor(s)
BIOS Version/Date American Megatrends Inc. 2101, 06/04/2009
SMBIOS Version 2.5
Embedded Controller Version 255.255
BIOS Mode Legacy
BaseBoard Manufacturer ASUSTeK Computer INC.
BaseBoard Model Not Available
BaseBoard Name Base Board
Platform Role Desktop
Secure Boot State Unsupported
PCR7 Configuration Binding Not Possible
Windows Directory C:\WINDOWS
System Directory C:\WINDOWS\system32
Boot Device \Device\HarddiskVolume1
Locale United Kingdom
Hardware Abstraction Layer Version = "6.3.9600.16408"
Username Jimbo-PC\JIMBO-PC\Jimbo
Time Zone GMT Standard Time
Installed Physical Memory (RAM) 8.00 GB
Total Physical Memory 3.25 GB
Available Physical Memory 1.87 GB
Total Virtual Memory 3.81 GB
Available Virtual Memory 1.94 GB
Page File Space 576 MB
Page File C:\pagefile.sys
Hyper-V - VM Monitor Mode Extensions Yes
Hyper-V - Second Level Address Translation Extensions No
Hyper-V - Virtualisation Enabled in Firmware Yes
Hyper-V - Data Execution Protection Yes

Some info for you....
OS Name Microsoft Windows 8.1 Pro with Media Center
Version 6.3.9600 Build 9600
Other OS Description Not Available
OS Manufacturer Microsoft Corporation
System Name JIMBO-PC
System Manufacturer System manufacturer
System Model P5Q-E
System Type X86-based PC
System SKU To Be Filled By O.E.M.
Processor Intel(R) Core(TM)2 Quad CPU Q9400 @ 2.66GHz, 2670 Mhz, 4 Core(s), 4 Logical Processor(s)
BIOS Version/Date American Megatrends Inc. 2101, 06/04/2009
SMBIOS Version 2.5
Embedded Controller Version 255.255
BIOS Mode Legacy
BaseBoard Manufacturer ASUSTeK Computer INC.
BaseBoard Model Not Available
BaseBoard Name Base Board
Platform Role Desktop
Secure Boot State Unsupported
PCR7 Configuration Binding Not Possible
Windows Directory C:\WINDOWS
System Directory C:\WINDOWS\system32
Boot Device \Device\HarddiskVolume1
Locale United Kingdom
Hardware Abstraction Layer Version = "6.3.9600.16408"
Username Jimbo-PC\JIMBO-PC\Jimbo
Time Zone GMT Standard Time
Installed Physical Memory (RAM) 8.00 GB
Total Physical Memory 3.25 GB
Available Physical Memory 1.87 GB
Total Virtual Memory 3.81 GB
Available Virtual Memory 1.94 GB
Page File Space 576 MB
Page File C:\pagefile.sys
Hyper-V - VM Monitor Mode Extensions Yes
Hyper-V - Second Level Address Translation Extensions No
Hyper-V - Virtualisation Enabled in Firmware Yes
Hyper-V - Data Execution Protection Yes