Screen flicker, start menu wont stay open

I’m having a problem with my computer that I just can’t seem to solve. I first noticed while online I am having trouble typing in the address bar. When I attempt to type the text will highlight or delete itself. For example it I type www. It will highlight or just show “.w”. Also the screen has a steady flicker. Typing in search boxes for yahoo or Google seems to work fine. I have also noticed that when I click on the start menu that it will not stay on. I comes on for a split second then closes. I gives me almost no time to open programs or anything else. Most drop down menus do the same. Another thing, anything highlighted in any program has a steady flicker.
What I done so far is to scan for a virus or malware with Spybot, Malwarebyte, Ad-Aware and AVG and so far everything is fine. I have done a System Restore, opened the IE with no add-ons and reset it to its default settings. I have run the System File Checker but I no longer have the XP disc so it won’t let me complete the scan. When I restart in safe mode everything appears to work although I can’t get online. I’m running Win XP SP3 Intel CPU, 2160 GHz, 2.00 GB of RAM.
Any help with this problem will be greatly appreciated. Thanks