You'd have to save a massive amount of mpegs to slow your computer down but it's easy to find them.
All mpegs have a file suffix of 'mpg' or 'mpeg' - The former is far more common.
Click on 'Start' and then 'Search'. If you're using XP you'll then be asked what type of files you're searching for. (That, by the way, is one of the reasons I hate XP. Too much 'nannying'!). Just select 'All'. (If you're not using XP you'll get straight to where you want to be anyway). Type *.mpg in the search field and click 'Search'. This will locate all files with the 'mpg' extension. Delete all those you don't want. Then repeat for *.mpeg.
Even after you've deleted them all, I suspect that you won't find any noticeable improvement in your PC's speed. There are loads of things which could slow it down and ABers won't be slow to suggest possible remedies - but it's usually simplest just to re-install Windows. If you need instructions, just put another question here but remember to state which version of Windows you use. (I can tell you how to re-install pre-XP Windows but I've been told that XP is slightly different).
Chris