If u Google, u will see lots of people have problem with this. Look at the Apple support site, lots of complaints, some suggested solutions, sometimes they work, sometimes they don't.
Answer: WIFI sync seems like a half-baked feature so far.
Specific to Windows I read: One thread blames a Windows App (forgot which one) is incompatible with WIFI sync. Solution: Reset the TCP/IP stack. Seems to have resolve the problem for some people, but not for others.
I used a Windows iTunes server and it mostly work.
I installed Bonjour for Windows.
At times, my AppleMobileService max out my cpu, I just kill it.
When WIFI sync stops working, I USB plug in my iPhone, uncheck the WIFI SYNC box, close iTunes, re-open, and re-check the WIFI SYNC box again, this seems to put me back in business. Don't ask me why, but that's what I do to keep it working.
When it works is great. I come home, plug in my charge cable, and as long as my server box is on and iTunes is loaded, it starts to SYNC the stuff my server box has downloaded for the day.
Needless to say, you want to run the latest version of iTunes. I believe WIFI SYNC started with 10.7.
My WIFI SYNC even when it works, doesn't seems to like synching more than once a day.