Massive variable in (full) scanned files

Hi, think I'm in the correct place!                  I have NIS 2011, (Packard Bell 2003,Windows XP home, SP3).

When I run a full scan th results vary massively. Some times it scans 284,380  ..and others 40,421.       See below:-

 

File Directory:          281,048       40,421

Rigistry                            271              266

process/start              2,477           2,296

netork/browser               278             578

other                                      6                 6

trusted                           1,663         1,630

Skipped                                0        30,500

 

Any ideas why?   P.S.  Norton Power Eraser and Malwarebytes  do not detect anything .