problem with postgres

Dear friends

I hope some one can help me

Server machine celeron 2.4Gb with 512 Ram
postgres 7.4.5
conectiva 8 with kernel 2.4.19

i´m getting many message ofErro:canceling query due to user request

how to solve this
i even get with an just booted up server, and running vacuum analyze verbose 
inside psql with no other people connected


the only changes i has on postgresql.conf is
max_connection=512
shared_buffers=8192
sort_mem=8192
vacuum_mem=31792

Regards
Luiz Rafael