Installing Ubuntu on Windows

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http://askubuntu.com – In Win Xp Firefox ,Google Chrome and IE crash very often (Due to some virus or something else). My friend suggested I should install Ubuntu Operating System (As it is unaffected by most viruses).Now My question is If I Install Ubuntu along with windows on the same Hard Disk - will it slow my computer even more because 2 Operating Systems are in the computer..or will it solve my problem. Also Should I install ubuntu 10 or ubutu 12.04 System Specs AMD Sempron processor 2.21 Ghz 790 MB of RAM (HowTos)