KARSTEN M. SELF, who oversees a children's computer lab at a youth center in Napa, Calif., spends about a half-hour each morning electronically scanning 10 PC's. He is searching for files and traces of code that threaten to hijack the computers by silently monitoring the children's online activities or by plastering their screens with dizzying - and nearly unstoppable - onslaughts of pop-up advertisements.

To safeguard the children's computers, Mr. Self has installed a battery of protective software products and new Web browsers. That has kept some - but by no means all - of the youth center's digital intruders at bay. "You would expect that you could use these systems in a safe and sane way, but the fact of the matter is that you can't unless you have a fair amount of knowledge, time to fix the problems and paranoia," he said.

The parasitic files that have beset Mr. Self and other frustrated computer users are known, in tech argot, as spyware and adware. The rapid proliferation of such programs has brought Internet use to a stark crossroads, as many consumers now see the Web as a battlefield strewn with land mines.