Web surfers battling "spyware" face a new problem: so-called spyware-killing programs that install the same kind of unwanted advertising software they promise to erase. Millions of computers have been hit in recent years by ads and PC-monitoring software that comes bundled with popular free downloads, notably music-swapping programs. The problem has attracted dozens of companies seeking to profit by promising to root out the offending software. But some software makers are exploiting the situation, critics allege, turning demand for antispyware software into a launch pad for new spyware attacks. A small army of angry Web users has set up a...