It is more than adequate. Providing you have a current AV (free one, don't pay) there is no point in spending money on another firewall especially if you have a router. The chances of a home computer getting hacked are the same as mine in winning the EuroLotto 3 weeks in succession.
There is a lot of hype and rubbish written about hacking home computers which spooks many users.
I used Comodo (free) on XP, found it good but a bit intrusive. Following favourable comments on the forums I now use (free) PC Tools on W7. Seems OK. Others are happy to use the built in one but I am not sure if it is so good at outgoing stuff.
I've used Windows 7 Firewall and Microsoft's Security Essentials without any problem and I can recommend both. Initially I checked with other products to make sure nothing was slipping through the MS products and after a few months all was clear.