There is no limit, I suppose, to the ingenuity of hackers. It's certainly possible hackers could have entered your Web space and modified files. Even the White House's Web site has been hacked many times.

There are several other possible ways your Web pages are modified without your knowledge: user error, ISP administration, and third-party browser add-ons.

Depending what you used to create your Web site, you may have inadvertently added links that you did not intend. For example, FrontPage offers a toolset that makes it easy for you but has a level of complexity that may create more problems than those you are trying to solve. Sometimes, those who are experimenting with the page layout may inadvertently leave parts of their experiments in the page when it goes online.

Your Web hosting provider, especially free sites such as GeoCities and Yahoo!, may reserve the right to add content to your pages. This is explicitly mentioned in the Terms of Service agreement and so, while you might not like it, you have in fact agreed to your page's modification.

Lastly, browsers and browser add-ons may add or even replace images and links on your pages. There is current litigation involving Gator and its propensity to modify Web pages. See this list of news reports provided by Google. There are other services where you enter a Web address and it returns that page, having converted nearly every word into a link that points to an online dictionary. Other browser add-ons that block popup windows, flashing graphics, in-line ads, etc., also modify pages before the browser renders them.

There is nothing one can do about pages that are modified after your hosting provider sends them on their way ("serves" the page). But for instances where your actual files have been replaced by others (a "hacked" site), your best defense is to regularly monitor your pages. There are programs that can make scheduled connections by FTP to your Web space and perform file comparisons. See FTP Voyager for that program's FTP Scheduler function. There are other FTP programs with this capability as well. I may be wrong, but I think Microsoft's FrontPage extensions are able to quickly determine if any files have been corrupted. You can also subscribe to services that compare files for you, such as Freshwater.com. Depends how critical your site's integrity is to you.