The best way to see the content of a valid http POST request is to look at the content of a request from a working program, like your browser.

Install an http tracer or debugger, which will log all http communications going out of your machine. Then, go to a site that has html for a form on it, click the button on site, and then look at the output from the tracer utility.

A quick Google search turned up two candidates:

HttpTracer from Lazy Dog Utilities at http://www.lazydogutilities.com/index.htm

Fiddler, a HTTP Debugging Proxy at http://www.fiddlertool.com/fiddler/

As a test, I quickly downloaded and installed HttpTracer, and set my browser (which is IE) to use it as a proxy (I set the proxy to 127.0.0.1 on port 8010, as explained in the Help file). On this page (i.e., the CodeGuru formu) the html includes a form with a "submit reply" and a "preview post" button. When I press the "preview post" button, HttpTracer logs the request as follows:

Code:
POST http://www.codeguru.com/forum/newreply.php HTTP/1.0
Accept: image/gif, image/x-xbitmap, image/jpeg, image/pjpeg, application/vnd.ms-powerpoint, application/vnd.ms-excel, application/msword, application/x-quickviewplus, application/x-shockwave-flash, */*
Referer: http://www.codeguru.com/forum/newreply.php
Accept-Language: en-us
Content-Type: application/x-www-form-urlencoded
Proxy-Connection: Keep-Alive
User-Agent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.0)
Host: www.codeguru.com
Content-Length: 1320
Pragma: no-cache
Cookie: pmt=1; bbuserid=85023; cguserid=MikeAThon; bbpassword=ffffd9b1fffffffffffffffdee88ffff; vbcodemode=1; bbsessionhash=ed4bb4b7f7e55fb47ec1bb75e4f627f0; bbthread_lastview=ax2x-ix339109ysx10x%221115052487%22yix338949ysx10x%221115074342%22y_; RMID=3fcfad0b42717e60

title=Re%3A+CHttp...+Post+with+%22action%22&mode=1&message=The+best+way+to+see+the+content+of+a+valid+http+POST+request+is+to+look+at+the+content+of+a+request+from+a+working+program%2C+like+your+browser.%0D%0A%0D%0AInstall+an+http+tracer+or+debugger%2C+which+will+log+all+http+communications+going+out+of+your+machine.++Then%2C+go+to+a+site+that+has+html+for+a+form+on+it%2C+click+the+button+on+site%2C+and+then+look+at+the+output+from+the+tracer+utility.%0D%0A%0D%0AA+quick+Google+search+turned+up+two+candidates%3A%0D%0A%0D%0AHttpTracer+from+Lazy+Dog+Utilities+at+http%3A%2F%2Fwww.lazydogutilities.com%2Findex.htm%0D%0A%0D%0AFiddler%2C+a+HTTP+Debugging+Proxy+at+http%3A%2F%2Fwww.fiddlertool.com%2Ffiddler%2F%0D%0A%0D%0AAs+a+test%2C+I+quickly+downloaded+and+installed+HttpTracer%2C+and+set+my+browser+%28which+is+IE%29+to+use+it+as+a+proxy+%28I+set+the+proxy+to+127.0.0.1+on+port+8010%2C+as+explained+in+the+Help+file%29.++On+this+page+%28i.e.%2C+the+CodeGuru+formu%29+the+html+includes+a+form+with+a+%22submit+reply%22+and+a+%22preview+post%22+button.++When+I+press+the+%22preview+post%22+button%2C+HttpTracer+logs+the+request+as+follows%3A%0D%0A%0D%0A&iconid=0&s=&do=postreply&t=338949&p=1146221&posthash=dc67ffed7dc20a849ede69da6e45dc59&poststarttime=1115086873&preview=Preview+Post&parseurl=1&emailupdate=1&rating=0
You can use this output to ensure that your own code matches.

You can also "see" what your code is sending out since the Wininet functions piggy-back on the proxy that you set into IE (at least I think they do).

HTH,
Mike