I have included a list below of what percentage of malware is distributed to Windows and their respective service packs.

Windows XP (RTM): 35%
XP SP1: 25%
XP SP2: 35%
XP SP3: 23%
Windows 2000 SP4: 7%
Windows Vista (RTM): 5%
Vista SP1: 3%
Vista SP2: 1%
Windows Server 2008: 2%
Windows 7: 23%

Updated Feb. 2011

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Malware comparatives:

Rootkits versus Viruses: 4%|22%
Crapware versus Viruses: 16%|19%
Trojans versus Viruses: 62%|23%
Bootkits versus Rootkits: 1%|64%

From this table it shows that Vista is relativly lower than what i previously thought.