Integrating Ubuntu with a Windows-based network is harder than it should be

I've been using and advocating free software for around six years. When studying and then working as a freelance writer, migrating an office seemed so simple -- draw up a list of comparable programs and, over a reasonable period, move your staff across. But over the past few weeks I've been trying to use Ubuntu Gutsy on my desktop PC in a Windows-based office, and whilst most things work just fine, it's far from the seamless integration I was hoping for.

I work for an environmental charity in London, and our office is pretty typical of the sector. Two Windows 2003 servers provide Exchange for email, calendaring, and contacts, along with some shared folders and printers, a PPTP VPN for remote work, and a VoIP phone solution called IP Office. On the desktops we use the typical software titles: Windows XP, Office 2002/3, some of the Adobe Suite (Acrobat Reader and Professional, Photoshop, Illustrator, Dreamweaver), Sage, and a handful of other programs.