Normally a "server at work" is treated as important...you should seek something officially supported and designed to work on one, and designed to keep maximum performance on a server (read this as not heavy/bloated/system slowdown). 20-40 bucks a year, and 15-30 per year for renewal..on something as important as a server....
First off avg free is not licensed for commercial use so even installing it on the workstations is piracy. Spend the money and either get the full avg(which they do have a server version of) or get something elce. I recomend kaspersky or eset nod32.
Clamwin will run on server but I really don't really recomend it. You need to have your work just spend some money on av protection.
wow, yeah, you can't install AVG Free on your server at work because its not designed or licensed to be installed on a server at all. Like swatbat said, it is not legal for any business to use AVG Free for desktops, let alone servers if it even would install. Ad-Aware Free is also not legal for business use. You will not have much luck finding free AV solutions that are legal to run in a business environment.
You will have to plunk down some cash and buy some legal software. Kapersky or NOD32.
If you've got multiple workstations and servers to protect, I'd look at eset's pricing for bulk licensing of NOD32. They give pricing discounts determined by the quantity of licenses you buy. You can often save quite a bit of money going that route. If NOD32's not your thing, other companies will usually do similar things with bulk licensing.