Well, yes it looks that Sygate is a problem. Their NDIS driver makes a dirty hook by modifying TCPIP.SYS module image (ArpSend routine if I remember fine). The only possible work around I could find is re-patching TCPIP.SYS after Sygate driver, but I really don’t like this solution. You can try to explain Sygate that dirty hacking is no good but I don’t think they would listen.
Latest WinpkFilter release also includes NDIS IM variant of WinpkFilter driver. Basically it was created to be used for Windows x64 (patching system is hardly possible there unless patching KeBugCheckEx in the kernel or something of this kind, see the notes from MS here http://www.microsoft.com/whdc/driver/kernel/64bitPatching.mspx). However, you can also use this driver for the latest operating systems (Windows 2000/XP/2003). By design it can coexists with Sygate driver, so it’s also a variant.