The simplest editor with UTF-8 support is Windows's built-in Notepad. In general it's fine; the only thing it's likely to trip up on is that it only supports Windows-style CR/LF line endings, making editing bare-LF files a pain.
Notepad is in all cases preferable to the dreadful WordPad, which is likely to mess up your files. The only reason WordPad used to be of use was that it could edit files longer than 64kB in Windows 9x. However Notepad has no trouble with long files in the WinNT series.
There are many, many editors that are a step up from Notepad; they will all give you syntax highlighting to make XML editing slightly clearer. Notepad++ is, notably, free. Generally though the syntax highlighting won't be enough to detect well-formedness errors so you'll still need something to test that, such as loading the file into Firefox, or, for Notepad++, the XML Tools. I used EmEditor with the valid XML checker plugin; EmEditor is a good little editor but it isn't free.
Then there are dedicated XML editors, like XMLNotepad. These will always generate well-formed XML, but the interface may be a bit more intimidating. Certainly they are more suited to data-driven XML than document-style markup.