iTunes (and Airfoil) can see Airport Express but can't connect in Windows XP (error -15000) - firewall problems?

I am using Windows XP Professional 2002 w/ SP3 on a Dell Latitude E4300 laptop wirelessly connecting to my Airport Express 802.11n (firmware 7.5.2).

I can see my Airport Express in iTunes 10.4.1 and can select it via the Airplay button. iTunes even detects whether or not a speaker is connected to the Airport Express. But when iTunes tries to connect, it comes back after a while saying "An error occurred while connecting to the Airplay device xxx. An unknown error occurred (error -15000)."

A similar problem happens with Airfoil from Rogue Amoeba that I use to stream music from other apps such as VideoLAN to Airport Express on the same laptop. I can see that my Airport Express is available but when Airfoil tries to connect, it comes back saying "It appears that a firewall on this computer is preventing Airfoil from connecting to xxx Airport Express. Please check your firewall settings and try again."

I've set both Airplay and iTunes as exception programs in my Windows Firewall setting, and even added TCP ports 5000 to 5005 and UDP ports 6000 to 6005 to the exceptions as recommended by Airfoil. I've also disabled Symantec Endpoint Protection that Ihave installed. The only thing I can't disable is McAfee Endpoint Encryption, since I also use the laptop at work and I don't have permission to disableMcAfee. I use this laptop to connect wirelessly with other devices such as HP wireless printers, NAS drives and other laptops on my home network and have never had any issues, so i think that this issue is specific to the Airplay software being blocked by some firewall (I am assuming Airfoil also uses Airplay).

I've tried this on different wireless networks and I have the same issue with Windows XP on my Dell Latitude E4300. I've also set up the network with no security, but the problem doesn't go away.

I also tried downgrading iTunes to 10.3.1 but won't work. I think it's an Airplay issue, not a specific iTunes issue, since I'm facing the problem with Airfoil as well.

How do I trace what is going on that is preventing iTunes and Airfoil from streaming to the AirportExpress within Windows XP?

I also have another Dell Inspiron laptop on the same network running on Windows Vista, and have no problems streaming music from iTunes or Airfoil to the Airport Express on the same network, even when the network is WPA-2 Personal (TKIP+AES) protected (I've seen other postings where WPA has been an issue, WEP has not, but in my case, this is not an issue). Ditto with my iPad2.

Thanks for your help!