my pc is very slow

it take as nearly 10 minutes to open a execel or word file.
mail merge is not working at all

i am not getting the required fields...means if there are 25 fields in the excel file then the mail merged file shows only 5 fields......
the mail merge is not successful.

Does the computer have any problems outside of Office programs?

You might try a System Restore to a point prior to the problem:

start > All Programs > Accessories > System Tools > System Restore

sir, the mail merge is also not working..when the original exel sheet has 25 fields , the merged document shows only 5 fields