By default Windows 7/Vista make use of only 512 Kb of your RAM as "Buffer Memory", to optimize exchanges between the Hard drive and the Main Memory.
If your system sports more that 1GB of RAM, then you can increase the space allocated.
For this you must make some modifications in the registry (better make up a backup before).
Click on Start/Run/regedit.
Expand the following key: HKEY_LOCAL_MACHINE/SYSTEM/CurrentControlSet/Control/Session Manager/Memory Management.
Search for the following value: IoPageLockLimit
In the case the value doesn't exist, create it:
Right click and choose New > DWORD Value (32bit).
Name the value IoPageLockLimit and double click it.
Select Hexadecimal (base).
Enter value F0000 and click OK.
Close the Registry Editor and restart your PC to apply the change.