The Department for Education and Science (DfES, UK) provides lots of free interactive teaching programs (ITP) which are great and safe to run or download. To play, you will need Adobe flash player or similar on your computer. You can download one from the same site if you need to.
There's an ITP for area and perimeter and lots of others. http://www.iwbmathstraining.co.uk/ol...0-%20dfes.html
On the homepage there is also a link to a guide you can download to explain how the ITPs can be used.