PostgreSQL 9.0 x64 bit pgbench TPC very low question?

I have server computer install Windows 2008R2, PostgreSQL 9.0.1 64 bit, 8G RAM, RAID 10 - 4 disks, dedicated server
Here is config of postgresql.conf after running pgtune

default_statistics_target = 100 # pgtune wizard 2010-12-15
maintenance_work_mem = 480MB # pgtune wizard 2010-12-15
constraint_exclusion = on # pgtune wizard 2010-12-15
checkpoint_completion_target = 0.9 # pgtune wizard 2010-12-15
effective_cache_size = 2816MB # pgtune wizard 2010-12-15
work_mem = 8MB # pgtune wizard 2010-12-15
wal_buffers = 32MB # pgtune wizard 2010-12-15
checkpoint_segments = 64 # pgtune wizard 2010-12-15
shared_buffers = 960MB # pgtune wizard 2010-12-15
max_connections = 254 # pgtune wizard 2010-12-15

After running pgbench
pgbench -i -h 127.0.0.1 -p 5433 -U postgres -s 10 pgbench
pgbench -h 127.0.0.1 -p 5433 -U postgres -c 100  -t 10 -C  -s 10 pgbench

Scale option ignored, using pgbench_branches table count = 10
starting vacuum...end.
transaction type: TPC-B (sort of)
scaling factor: 10
query mode: simple
number of clients: 100
number of threads: 1
number of transactions per client: 10
number of transactions actually processed: 1000/1000
tps = 20.143494 (including connections establishing)
tps = 256.630260 (excluding connections establishing)

Why pgbench on my server is very low or is it common value with my server ?

Please help me. Thanks in advance.

Tuan Hoang ANh.