http://www.pidgin.im/
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You wouldn't need to use any tools to defrag, to scan for malware or to repair the registry. So, not being able to use those Windows apps is not a drawback. There are antivirus apps for Linux, but it has very few viruses to detect. This is a list of known viruses that can affect Linux, but most of them are obsolete and Ubuntu 11.04 is immune to most of them: http://en.wikipedia.org/wiki/Linux_malware#Threats