How can I cover old and yuck carpet with a non-allergenic alternative without ripping up the existing carpet?

Question:Our landlord wont let us replace the carpet but it's old and yuck! Way beyond the help of steam-cleaning, I cant begin to imagine the allergens living in it. Is there a way we can cover it with something smooth its not so dusty?

Answers:
Yikes, adding anything on top of carpet is bad! Especially if you are concerned about allergens. Another layer will just trap more moisture and allow more breeding by you-don't-wanna-knows.
Usually a landlord will balk at replacing a carpet but jump at the chance to have tenants replace carpet themselves.
Push!