Why Long Internet Use Triggers System Screw-Up?

Perplexed!! A few weeks ago, I noticed some odd performance events after being on the internet for awhile. Now, it has gotten much worse. I have a HP computer with XP Pro. Here are the symptoms:
1. It begins after actively using the internet for 2 or 3 hours.
2. Text and graphics start to blank out in various places on the screen page. If you hit refresh, the blank areas move around and about. Also occurs when moving the cursor around page. The minimize sessions listed at the bottom of the screen often blank out. Efforts to do anything give unpredictable displays.
3. In the beginning it seemed that deleting internet temp storage using "tools" helped eliminate the problem but now, when possible to use, it doesn't seem to always work. Sometimes that delete option window starts to be unresponsive and blanking out.
4. All system restore points are gone as if wiped out. After I reboot, I can set a new restore point and it and any others set stays on until the problem reoccurs again - then gone!
5. There have been a couple of times when I go to shut down, the word "start" at the bottom left is gone and when I can click on the "turn off" option, I get a corrupted "stand by", "turn off", "restart" window. If I am able to click on one, the system may not respond and I have to simply power off.
6. When I reboot, THE PROBLEM IS GONE except for seeing the desktop icons mentioned next.
7. My desktop gets a series of odd icons added to it. The number can vary from 3 to 10. Each one has a square at the top with a smaller square in the center with what appears to be extremely tiny icons inside (can't really identify). Under that larger main square, there is a series of smaller squares in two rows that vary in number. There may be 5 in one row and 8 in the second row. When I click on any one I get an "Open With Windows" window, listing a file name of the icon squares!
8. I ran an AVAST scan and found two items listed. The first was simply a statement about insufficient storage. I couldn't click on it. The second was a setup file name identified with a virus named "Sality". On it I was given several options so I selected the "repair" one and it came back complaining about insufficient storage. So I then selected the quarantine "chest" option for the file. It worked but did not fix the problem.
9. I ran some Malware programs and found no issues. Ran CCleaner to remove listed temporary storage types (but was surprised to see a large number of used storage there).
10. Looked at both of my disk drive properties and both have a HUGE amount of free space. I have 1024 RAM.
11. I ran windows Chkdsk repair disk program on both of my disk drives and it found no problems except some minor index references.

It appears if I just reboot after every couple of hours of internet use, the problem will not occur for another 2-3 hours.
What is reboot doing to temporarily fix it? Any ideas appreciated.