Cannot Open This File; Rundll32.Exe!

Dear Folks,
After having done some web surfing activity, I just lost all computer files extension code [like my computer, control panel, system tools,registry, web browser, Internet explorer, Microsoft office software {word, exel, PowerPoint, acess etc}]; even it knocked out the Avast virus software too. They all look like worms file; and whenever I click any of them; the message pops up by showing that,
Windows cannot open this file;
File: rundll32.exe
To open this file, windows need to know what program created it, windows can go online to look it up automatically or you can manually select from a lost of programs on your computer,
What do you want to do?
Use the web service to find the appropriate program
Select the program a list.
Ok cancel
Under above those circumstances, how do I recover the fatal problem?
Should I do the repair installation for WinXp pro?
I will be waiting for your important reply!