Just to clarify - a copy of Microsoft.SharePoint.dll will allow you to reference and compile against it but it won't actually work unless its running on a machine with SharePoint properly installed.
A few options for you.
You can download, for free Windows SharePoint Services 3.0 - but it needs to run on a Windows Server OS (2003+).
You can download (for free again) Windows Server 2010 Foundation and get it running on Windows 7 64bit client OS for a development environment.
You can download a 30 day trial of MOSS on a preconfigured VHD - basically a complete pre-configured virtual server - but you need Hyper-V or VMWare Server running on a 64 bit CPU to run this virtual server.
There is a hack to get WSS running on Vista - but its unsupported and the licensing/legal situation is unclear - very probably its in breach.
You could use SharePoint's web services then you won't need the dll or access to the SharePoint server apart through the web services - which assuming you can browse it you should be able to get.