Programs that You can Safely Open Files With

Don't ever use Outlook Express, Internet Explorer, Microsoft Word, Microsoft Office, Microsoft Media Player, Real Audio, WinAmp or QuickTime to open files that you don't fully trust.

Alleged File Type Programs that are Safe to Use
Text (*.txt) Notepad or WordPad
Documents (*.doc, *.wri, *.rtf, *.wps) Write, WordPad, and Microsoft Works. Do not use Word.
Spreadsheets Microsoft Works. Do not use Excel.
Zip (*.zip) Windows XP itself, or WinZip SR-1 (Aug. 26, 2004 -- has important security-related changes.)
PDF (*pdf) The latest version of Adobe Reader is probably safe. Earlier versions are not.
HTML (*.htm, *.html) 1. Notepad, Write or WordPad: The message may be confusing to read, but you should be able to make out the gist of it.

2. The tiny "OffbyOne" [free] browser is the best tool for this purpose. It's safe because it's a passive browser.
Images (*.jpg, *.jpeg, *.gif, *.bmp, etc.) Use Microsoft Paint [Start > Accessories > Paint].
Video (*.mov, *.avi, *.wmv, etc.) I don't know of any programs that are truly safe for files you don't trust. There are some precautions you can take though.
Executables (*.exe, *.pif, *.com, *.cmd, *.vbs, etc.) Are you serious? There are some "sandbox" programs that you could use, but that's beyond the scope of this process.
Music (*.ra, *.mp3, *.wma, etc.) I don't know any programs that are safe for this purpose. You could download *.mp3 files and play them safely on your portable MP3 Player though. :-)
Unknown or unfamiliar file types It's safe to open any file from Notepad or WordPad.