|How many things are running in the background? |
Too much running will cause the sort of problems you describe.
To see what is starting up when Windows boots, go to Start > Run, and type in "msconfig" (without quotes), then press OK. Go to the Startup tab and uncheck everything, with the exception of anti-virus/anti-malware and firewall.
By default, Windows has a few services running in the background that are not needed by the average PC user.
To gain disable some of these services, go to Start > Run, and type in "services.msc" (no quotes), and press OK. For a list of services and configuration recommendations, go to www.blackviper.com, and select the config page for your OS and service pack. I disable a few that he doesn't recommend, but only because I know I don't need them in my situation. Unless completely certain, don't disable any that he doesn't suggest.
I also like to let Windows manage the size of my pagefile/virtual memory. You can google instructions for this (if you decide to try it).
I also like to make regular use of CCleaner to remove junk files and unneeded registry entries.
Update your virus definitions and do a complete system scan, cleaning/removing any infections. If you have no anti-virus (a silly thing if you access the internet) download and install AVG Free. Be careful with the download, as there is also a paid version, but the free one is as good as (or better than) most paid-for anti-virus programs.
If you follow my above instructions, you'll see a noticeable performance increase, and hopefully your problem with movies will be resolved.
If performance is still an issue, I'd suggest the addition of more RAM (you are running with what many consider to be the bare minimum). It is one of the cheapest upgrades you can do, bang-for-buck.
If you need further help, post back. There are a few other possible causes for your problem, but you should try improving performance first (more benefits than just smoother playing movies).
Please let us know if you found someone's advice to be helpful.