2003 32bit. Updating to 2008 isn't an absolute requirement, but now is the time to try it and if an actual upgrade works, then I'm at least it's on a supported OS, for now. Since it's 32Bit, I won't be able to upgrade past 2008. (The host for the VM is 2016.)
All fun stuff.
You may want to install the lowest version of IIS (assume YPI use that) you can possibly put on it. Or maybe bring up a 64 bit Win 2008 and bring the site over.
At work couple days ago we try to make an ASP.NET 2.0 applications that was on Win Server 2012 to work in Win 2016 on an Azure VM. For reason nobody can ever explain it cannot connect to the SQL Server!! The ASP.NET 4.0 applications are fine, just that antique that was the main application. Ended up converting a copy of the Win 2012 VM to Azure.
All the UI rendering codes probably hard to convert to anything modern even if you want to try. Most of the converters are long gone by now but there were some in older Visual Studios that claimed to be able to port those classic ASP to probably ASP.NET with VB.NET code behind. Forget that for now until you have everything else running in your servers.