Hi Clair,
You should Not have to buy anything to watch a movie download.
If you are redirected to another site asking for you to buy a Specific player, then the File is either Corrupt, a Virus, or Spam.
When downloading Video Files,
ALWAYS use Bitzi. See Instructions Below.
After you have completed your search, Right click on the file that you're interested in. Now go to Advanced > Lookup File With Bitzi. Left click on that and a new browser window will open which is Bitzi. Now you will see how other members have rated this file. If you see something like "Bitprint not found" then Leave it alone. Its probably a Fake or Corrupt File. If it has been Rated then you can read what or how the members feel about the file.
If I'm off base above, then you may just need to download a Codec Pak. See Below.
First off I would suggest you do a Google Search to get an understanding of what Codec's are and what function they perform. K-lite Codec Paks, and CCCP are probably some of the best, but it may depend on your OS.
Just keep in mind that using more than 1 Codec Pak will cause Problems.
There is a MS WMP Codec Pak included in Link Below.
See here for suggestions By LOTR. Hope this Helps.
CODECs list for Movies/Audio/ Alternate players / Video help forums/ video-mp3 fixers
Hope this Helps.