I don't know if this is correct but here is what appears into the call stack window:

 	>	php5ts.dll!zend_get_executed_lineno(void * * * tsrm_ls=0x015ee1a0)  Line 367 + 0x2 bytes	C
 	php5ts.dll!zend_error(int type=2, const char * format=0x00a60e3c, ...)  Line 930 + 0x8 bytes	C
 	php5ts.dll!sapi_header_op(int op=0, void * arg=0x0175fa00, void * * * tsrm_ls=0x015ee1a0)  Line 533 + 0xf bytes	C
 	php5ts.dll!zif_header(int ht=1, _zval_struct * return_value=0x013c2d98, _zval_struct * * return_value_ptr=0x00000000, _zval_struct * this_ptr=0x00000000, int return_value_used=0, void * * * tsrm_ls=0x015ee1a0)  Line 49 + 0x17 bytes	C
 	php5ts.dll!zend_do_fcall_common_helper_SPEC(_zend_execute_data * execute_data=0x0175fa98, void * * * tsrm_ls=0x015ee1a0)  Line 192 + 0x35 bytes	C
 	php5ts.dll!ZEND_DO_FCALL_SPEC_CONST_HANDLER(_zend_execute_data * execute_data=0x00000000, void * * * tsrm_ls=0x015ee1a0)  Line 1587 + 0xe bytes	C
 	php5ts.dll!execute(_zend_op_array * op_array=0x00000000, void * * * tsrm_ls=0x00000000)  Line 92 + 0xc bytes	C
 	msvcrt.dll!77c39f8e() 	
 	[Frames below may be incorrect and/or missing, no symbols loaded for msvcrt.dll]	
 	user32.dll!77d510c7() 	
 	user32.dll!77d5110e() 	
 	php5ts.dll!php_execute_script(_zend_file_handle * primary_file=0x0175fdcc, void * * * tsrm_ls=0x015ee1a0)  Line 1720 + 0x12 bytes	C
 	php5apache.dll!apache_php_module_main(request_rec * r=0x007bd2f8, int display_source_mode=0, void * * * tsrm_ls=0x015ee1a0)  Line 53 + 0x1b bytes	C
 	php5apache.dll!send_php(request_rec * r=0x007bd2f8, int display_source_mode=0, char * filename=0x007bde68)  Line 643 + 0xb bytes	C
 	php5apache.dll!send_parsed_php(request_rec * r=0x007bd2f8)  Line 657 + 0xe bytes	C
 	ApacheCore.dll!6ff64bc7() 	
 	ApacheCore.dll!6ff74a73() 	
 	ApacheCore.dll!6ff74746() 	
 	ApacheCore.dll!6ff6be0f() 	
 	ws2_32.dll!71ab3a91() 	
 	ApacheCore.dll!6ff6bc5a() 	
 	msvcrt.dll!77c3a3b0() 	
 	ws2_32.dll!71ab3a91() 	
 	ws2_32.dll!71ab87a9() 	
 	kernel32.dll!7c80b50b() 	
 	ws2_32.dll!71ab3a91() 	
 	ws2_32.dll!71ab87a9() 	
 	kernel32.dll!7c8399f3() 	


for the following error:

The thread 'Win32 Thread' (0x13d4) has exited with code 0 (0x0).
Unhandled exception at 0x00876d6c (php5ts.dll) in Apache.exe: 0xC0000005: Access violation reading location 0x0001ee6c.


with this build: php5.1-win32-200601110730.zip

Now I am able to understand better the problem after studying the manual.

http://www.php.net/virtual
" To run the sub-request, all buffers are terminated and flushed to the browser, pending headers are sent too."

This means that the headers are sent to the browser when virtual() is used.

Now, my code from the a.php beeing:

<?php virtual('/b.php'); ?>
<?php header('Location:c.php'); ?>

The normal PHP error message should be on both Linux and Windows machines:

Warning: Cannot modify header information - headers already sent by (output started at /var/www/html/a.php:1) in /var/www/html/a.php on line 2

Windows Bug: Right now the PHP on Windows crashes (by seeing the trace), I suppose, when tries to get the line number either where the headers were sent or where the Warning is generated.

Linux Bug: The error line that is reported to generate the Warning is 0 instead of the line where the header() statement is used meaning line 2.

Please try using this CVS snapshot:

  http://snaps.php.net/php5.1-latest.tar.gz
 
For Windows:
 
  http://snaps.php.net/win32/php5.1-win32-latest.zip

I can not reproduce this bug.

No feedback was provided for this bug for over a week, so it is
being suspended automatically. If you are able to provide the
information that was originally requested, please do so and change
the status of the bug back to "Open".