Windows XP has a built-in feature called Compressed Folders that lets you work with ZIP files such as those created by the $29 WinZip or similar programs.

However, unlike WinZip, if you attempt to open a file located inside a compressed folder, Windows will open it as read-only. To get read-write access, you must drag the file from the compressed folder into a "normal" folder (such as the Desktop).

The way this came up is that I sent a customer an Access mdb (zipped) in an email, and when they received it (on their new Win XP machine sans WinZip) they simply double-clicked on the Zip attachment, then double-clicked on the mdb file - and found that it was read only.