The software that plays YouTube videos is Adobe Flash Player.

The problem could be software in your PC, but it's equally likely to be congestion somewhere in the network between your PC and YouTube's servers. Try checking the speed of your broadband connection at Speedtest.net or similar. Also try YouTube at different times of the day and - if there's a systematic variation it it likely to be network congestion when everybody else is viewing and downloading stuff, not your PC.

Poor performance on YouTube is a common problem. You're not the only one.

There's a new version of Flash player out recently - might be worth upgrading... see Adobe Flash Player