Query planner problem

Hi,

I've moved a database onto a much faster machine but for some reason the
query planner is opting to use seq scan methods rather than the indexes.

This is a very large database without the indexes the performance is dire -
the new machine is a 2Ghz P4 with 1GB RAM, the old machine is a 900Mhz P3
with 512Mb Ram.

Any idea why this is happening adn what I can do to solve the problem ? I've
tried dropping & rebuilding the indexes, vacuum analyze etc but to no
effect.

Cheers,

Bob