"Could not open relation XXX: No such file or directory"

I am running Postgres 8.1.4 on Windows XP Professional Service Pack 3.
 
Yesterday when attempting to select from one particular database I repeatedly encountered errors such as the following:
 
2009-08-18 10:49:54 ERROR:  could not open relation 1663/51796/1247: No such file or directory
2009-08-18 10:49:54 STATEMENT:  SELECT typname FROM pg_type WHERE oid = 1043
 
1247 is pg_type. In Explorer I could see the file 51796/1247, and the timestamp indicated that it had not recently been updated. I had never previously seen this error.
 
As that database is used only for testing, I dropped it and loaded a new one from a dump. However, earlier today a colleague working against this new database experienced a slightly different error on several occasions:
 
2009-08-19 03:06:45 ERROR:  could not read block 0 of relation 1663/52752/52896: No such file or directory
2009-08-19 03:06:45 STATEMENT:  SELECT * FROM "RP"."RP_ATTRIBUTE" WHERE rp_id = (('MM100')::text)
 
52752/52896 is the Primary Key index on the referenced table. Again, I can see the file in Explorer, and it has not been updated since the database was created. When I try the same Select statement now I get no error.
 
Clearly something is amiss, but I don't know what. I should be grateful for any suggestions as to what I should check.