See this thread on the BOINC forums for all the information.
In short, BOINC exits when it is up- and downloading tasks. This is on a Business edition of Windows Vista with BOINC 5.10.20 .. It's installed in its own directory, run as administrator. BOINC just quits without warning.
The BOINC Manager window is open, but all the tabs show no data, and after a while, a messagebox appeared saying "BOINC Manager is not able to connect to a BOINC client. Would you like to try to connect again?". I press Yes and the tabs populate again and things start running again.
2007-09-28 08:21:27 [rosetta@home] Sending scheduler request: To fetch work
2007-09-28 08:21:27 [rosetta@home] Requesting 40 seconds of new work, and reporting 1 completed tasks
2007-09-28 08:21:32 [rosetta@home] Scheduler RPC succeeded [server version 509]
2007-09-28 08:21:32 [rosetta@home] Deferring communication for 4 min 2 sec
2007-09-28 08:21:32 [rosetta@home] Reason: requested by project
2007-09-28 08:21:34 [rosetta@home] [file_xfer] Started download of file vvv001_03_05.200_v1_3.gz
2007-09-28 08:21:34 [rosetta@home] [file_xfer] Started download of file vvv001_09_05.200_v1_3.gz
To pause/resume tasks hit CTRL-C, to exit hit CTRL-BREAK
2007-09-28 08:45:47 [---] Starting BOINC client version 5.10.20 for windows_intelx86
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2007-10-10 10:03:32 [boincsimap] Sending scheduler request: To fetch work
2007-10-10 10:03:32 [boincsimap] Requesting 47 seconds of new work
2007-10-10 10:03:37 [boincsimap] Scheduler RPC succeeded [server version 511]
2007-10-10 10:03:37 [boincsimap] Deferring communication for 7 sec
2007-10-10 10:03:37 [boincsimap] Reason: requested by project
2007-10-10 10:03:39 [boincsimap] [file_xfer] Started download of file 7100204.030123
To pause/resume tasks hit CTRL-C, to exit hit CTRL-BREAK
2007-10-10 10:16:39 [---] Starting BOINC client version 5.10.20 for windows_intelx86