Two new worms are exploiting a flaw that Microsoft patched last week and it appears separate hacking groups are competing to infect machines.

The first worm, Zobot C, is similar to the worms that appeared yesterday, but has been augmented so that it can spread via email by pretending to be a photographic attachment as well as by network sharing. It turns infected machines into part of a remote controlled botnet and is spreading slowly but steadily.

The second, Tilebot-F, is spreading much more quickly and downloads spyware as well as a Trojan allowing remote control of the PC.

"I suspect Tilebot has been written by a different group," said Graham Cluley, senior technology consultant for Sophos.

"It now looks as though separate gangs of virus writers are getting in on the act. It's like a gold rush and everyone is racing to catch the unpatched computers."

Major antivirus vendors have identified signature files and they are available for download and Microsoft is urging all users to patch systems immediately.