For being in a cold room I wear Ugg boots while working at my computer (which is most of the time). I even have poor circulation and they keep me warm.

Also there is a lot of heat loss from the top of your head, so wearing a hat will help keep the rest of you warmer too.

Occasionally, I will also put a small throw over my lap when it is colder, or sometimes I wear long underwear. I have been thinking of getting some flannel-lined jeans (LL Bean has them).

I don't use it, but our cats love to sleep on a an electric throw blanket. One of these on your lap ought to keep you plenty warm. I just saw these at Walmart for around $30.