MySQL replication can work over a WAN and is likely your simplest solution. My main concerns (other than running MySQL on Windows for something important enough to need failover) would be making sure the sites have enough bandwidth, and deciding whether to implement compression and/or encryption. I don't have a specific guide to recommend, but they are easy enough to find.
Windows multi-site clustering would be more difficult. You'll need block-level replication of storage and probably high throughput, low latency networking between sites if you want to fail over in a timely manner. Also, the MySQL installer does support Windows clustering now, but I don't know anyone actually running MySQL on a Windows cluster, and I don't know if it was tested on multi-site clusters.
Personally, if I were tasked with improving reliability of MySQL servers running Windows, I'd want to re-evaluate why Windows was chosen as an OS. Windows is perfectly capable of running highly available systems, but historically MySQL has had a number of deficiencies when running on Windows (bugs, resource usage limits, missing features and optimizations), and the MySQL development and support communities are far more active on Linux than Windows. This has improved somewhat since MySQL 5.5 when it was no longer compiled against the Windows POSIX API, but I'm not convinced that MySQL on Windows is at parity with MySQL on Linux. I know this isn't what you asked, but it's hard to honestly answer the question without bringing it up.