I have a big problem.
everytime I want to use a plane I have downloaded from the internet,
my FS5 says there is something wrong with the fule .....fso.

Where can I get these files ?
The error diagnostic about missing FSFS.FSO likely means you have
FS5.1 and you're trying to use an add-on aircraft built with Flight
Simulator Flight Shop (FSFS)without having this additional commercial
software on your system. FSFS is available from mail order advertisers
here at flightsim.com (~$50) and also in many computer stores.
Other options: Seek out "non-FSFS" aircraft. They're relatively
rare compared to the wealth of FSFS planes, but they require only FS5.1.
Another is to buy a later version of MS FS. With FSW95 or FS98,
you don't need FSFS. Instead there are (free) converters that work
with many (but not all) FSFS designs. However, you don't have the
aircraft building capability or audible programmable Air Traffic Control
features of FSFS.
Hope this quick review is helpful. -- Dennis S, FSEngEd@AOL.COM