well,ill recommend you some dell computers.they usually dont weigh much.just like 15 kgs max.
dell inspiron 580 is a slim design and weighs only 16.1 pounds.

http://www.dell.com/us/p/desktops#facets=51795~0~11384532,51795~0~11384531,135390~0~6687268,51805~0~812582&p=1

it is also well under your budget.549$

i recommend that you go and customize it.
it will increase 60 dollars and will increase 2 gb of memory.

now a pc with 8 gigs of memory and a core i5 will be super fast for converting,editing videos etc etc.

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