My indexes aren't being used (according to EXPLAIN)

Greetings:

Apologies if this question has already been answered, but I was unable
to locate a prior answer in the archives...

I have a table with approximately 10 million records, called
"indethom", and with an INTEGER column called "clavis" which is set up
as a primary key. When I try to perform a select on the table,
restricting the result to only the first 100 records, PostgreSQL
performs a sequence scan, rather than an index scan (as shown by using
EXPLAIN). Needless to say the sequence scan takes forever. Is there
some way to get PostgreSQL to use my wonderful indexes? Have I somehow
built the indexes incorrectly or something?

Here's the description of the table:

====================== PSQL Output Snip =========================

it=> \d indethom
Table "public.indethom"
Column | Type | Modifiers
---------------+-----------------------+-----------
numeoper | smallint | not null
nomeoper | character(3) | not null
... (numerous columns skipped) ...
verbum | character varying(22) | not null
poslinop | integer | not null
posverli | smallint | not null
posverop | integer | not null
clavis | integer | not null
articref | integer |
sectref | integer |
query_counter | integer |
Indexes: indethom_pkey primary key btree (clavis),
indethom_articulus_ndx btree (nomeoper, refere1a, refere1b,
refere2a, refere2b, refere3a, refere3b),
indethom_sectio_ndx btree (nomeoper, refere1a, refere1b,
refere2a, refere2b, refere3a, refere3b, refere4a, refere4b),
it_clavis_ndx btree (clavis),
verbum_ndx btree (verbum)

it=> explain select * from indethom where clavis < 25;
QUERY PLAN
----------------------------------------------------------------------
Seq Scan on indethom (cost=0.00..1336932.65 rows=3543991 width=236)
Filter: (clavis < 25)
(2 rows)

================== End Snip =====================

Feel free to point me to any FAQ or previous message that already
answers this question. Thanks in advance!

-Erik Norvelle