You must do a search for this topic (DVT - Deep Vein Thrombosis), I read a really long thread about this a while back.
There were quite a few well informed people discussing it, even people who had done medical research on Asprin use (Pandora I think) it was very good. Haven't got the time to find it for you, sorry.
Apparently Pilots have died from this, if I remember rightly.
Too much asprin rots your stomach lining away, so not a good long term solution. Drink plenty water, exercise your lower limbs regularly, even "fidget" your legs whilst sitting.
[This message has been edited by Princess PP (edited 21 March 2001).]