I set following keys:

(1) log["C:\Documents and Settings\Administrator\_log\sample.log"]
(2) log["C:\Documents and Settings\Administrator\??????\_log\sample.log"]
(3) logrt["C:\Documents and Settings\Administrator\_log\sample.log"]
(4) logrt["C:\Documents and Settings\Administrator\??????\_log\sample.log"]

only (4) doesn't work.

here is a agent debug log

3064:20120126:224422.289 In process_logrt() filename [C:\Documents and Settings\Administrator\??????\_log\sample.log] lastlogsize [0] mtime [0]
3064:20120126:224422.289 In split_filename() filename:'C:\Documents and Settings\Administrator??????_log\sample.log'
3064:20120126:224422.289 split_filename() C:\Documents and Settings\Administrator??????_log\sample.log
3064:20120126:224422.289 split_filename() ^
3064:20120126:224422.289 In split_string(): str [C:\Documents and Settings\Administrator\??????\_log\sample.log] del [\sample.log]
3064:20120126:224422.289 End split_string(): part1 [C:\Documents and Settings\Administrator??????_log] part2 [sample.log]
3064:20120126:224422.289 End split_filename() directory:'C:\Documents and Settings\Administrator??????_log\' format:'sample.log'
3064:20120126:224422.289 cannot get entries from [C:\Documents and Settings\Administrator??????_log] directory: [2] No such file or directory
3064:20120126:224422.289 Active check [logrt["C:\Documents and Settings\Administrator\??????\_log\sample.log"]] is not supported. Disabled.
3064:20120126:224422.289 In process_value() key:'windows:logrt["C:\Documents and Settings\Administrator\??????\_log\sample.log"]' value:'ZBX_NOTSUPPORTED'