HTTP 403 error when uploading spreadsheet to WSS 3.0

I recently setup a WSS 3.0 site and everything works fine except for creating a list by uploading a spreadsheet. After browsing to the desired spreadsheet and pressing "Import", I get "The website declined to show this webpage - HTTP 403; Most likely causes: This website requires you to log in." This is the second WSS site I've setup and both have the same issue. I have double-checked the browser's settings for security and using Windows Authentication. Internet searches don't come up with many issues regarding this error when uploading spreadsheets. Any ideas would be appreciated.