When you start your browser, the home page has mysteriously changed. You change it back manually, but before long you find that it has changed back again.

You get pop-up advertisements when your browser is not running or when your system is not even connected to the Internet, or you get pop-up ads that address you by name.

Your phone bill includes expensive calls to 900 numbers that you never made-probably at an outrageous per-minute rate.

You enter a search term in Internet Explorer's address bar and press Enter to start the search. Instead of your usual search site, an unfamiliar site handles the search.

A new item appears in your Favorites list without your putting it there. No matter how many times you delete it, the item always reappears later.

Your system runs noticeably slower than it did before. If you're a Windows 2000/XP user, launching the Task Manager and clicking the Processes tab reveals that an unfamiliar process is using nearly 100 percent of available CPU cycles.

At a time when you're not doing anything online, the send or receive lights on your dial-up or broadband modem blink just as wildly as when you're downloading a file or surfing the Web. Or the network/modem icon in your system tray flashes rapidly even when you're not using the connection.

A search toolbar or other browser toolbar appears even though you didn't request or install it. Your attempts to remove it fail, or it comes back after removal.

And the final sign is: Everything appears to be normal. The most devious spyware doesn't leave traces you'd notice, so scan your system anyway.