Particularly ones with a higher bitrate and flash movies. I know it's quite an old one and only 1Ghz but upgraded to 768mb RAM, it used to deal with movies fine but now it's struggling. Anyone got any ideas?

btw it's never been formatted and might be a bit clogged up, so any useful utilities to clean it up might help if there are any suggestions.

It doesn't have the raw power to play those files.

Just format and reinstall for best results.