The problems with the folder may be due to not having the correct or corrupt codecs for video files. It could also be affecting media player.
But if you havnt got any antivirus anyway i'd suggest the following:
I'm guessing your running XP?
Shut it down, restart into safe mode by pressing F8 and selecting safe mode from the menu.
Once started, click start, then click run and type:
press enter and you'll get a utility window.
click the startup tab and untick everything in there that is ticked (most lame viruses/trojans can be stopped from starting up doing this)
Click apply and ok, then restart the computer normally.
Once back on, download antivirus like AVG
and anti-spyware such as Ad-Aware
I suggest these as they are free and relatively small to download, simple to use and simple to install.
Scab you computer using both of these and hould it find any viruses or spyware, it will remove them for you.
Restart and then download and install ACE Mega codecs pack
(it had many codecs to view many videos) it'll probably as you to restart again.
Then try it and see if you have the same problem.