Hi,
The issue is likely to be caused by third-party programs or viruses.
I suggest trying the following steps to troubleshoot the issue:
Clean Boot
========
1. Click Start, type "MSCONFIG" (without the quotations) in the Search Bar and Press "Enter" to start the System Configuration Utility.
Note: Please click Continue if the "User Account Control" window pops up.
2. Click the "Services" tab, check the "Hide All Microsoft Services" box and click "Disable All" (if it is not gray).
3. Click the "Startup" tab, click "Disable All" and click "OK".
4. Restart the computer and test the issue.
Note: Clean Boot is a troubleshooting step. If some programs have been disabled, we can re-enable them later. If you see the System Configuration Utility, check the box of "Don't show this message" and then click "OK".
Please test this issue in the Clean Boot environment. If the issue disappears, we can use a 50/50 approach to quickly narrow down which entry is causing the issue.
If the issue persists, please update your anti-virus program and scan for viruses.
For the reason why only 3GB memory is recognized by your system, you can refer to the following article:
The system memory that is reported in the System Information dialog box in Windows Vista is less than you expect if 4 GB of RAM is installed
http://support.microsoft.com/kb/929605/en-us
In addition, we usually set the virtual memory 1.5 times as big as the physical memory.
Hope it helps.
Best regards,
Tim Quan
Microsoft Online Community Support