A fast-spreading computer worm corrupted global computer networks yesterday causing some Irish firms to shut down their systems and delays for many computer users. The worm, which is called "Nimda" first appeared in the US on Tuesday and had spread to thousands of European businesses by yesterday morning. It spreads by sending infected e-mails which carry an attachment labelled "readme.exe". It also propagates by infiltrating unsecured websites and attaching itself to computer users' internet browsers. Dublin-based life assurance firm, Friends First, was forced to shut down its computer systems yesterday afternoon after the worm was found to have infected at least four computers. A Friends First spokesman said the company shut its systems down as a precaution and it did not expect any collateral damage would be caused.