Unable to open files with double click

Hello,

I just scanned and removed all infected files on my system having XP OS using avast
After doing that on double clicking some of my files (xls,doc), it pops up the window asking me to find a program to use. i removed the mountpoints2 from the registry and for the 1st time the problem seemed solved. However the problem exists on restarting. The system has been totally scanned to remove the viruses.

Please help me here. Do i need to reinstall the OS or OS/applications ? Or is there any simpler way out.

Appreciate the help. Thanks