I'm no expert on this, but know enough to think that unless you are, you may want to avoid this.
The VPN connections on SME will mostly use IPSEC security, Windows 2003 (without add ons) doesn't.
In my (limited) experience, creating VPNs can be interesting enough when both ends of the tunnel are the same, if they're different, you can have all sorts of 'fun' trying to match the settings. Of course its possible & I'm sure there are lots of folk in this forum who can do it, but it depends on how you value your time and the 'learning opportunity'.
Could you find an old PC to build as an SME server at the Win2003 end & then use one of the well documented SME contribs at both ends? Alternatively, as a pair of decent VPN routers are likely to cost around £250 - £300 (GBP), is this an expence that you can justify?