Cannot get Service Packs to Update

I have an Acer Aspire One netbook running Windows xp.
I need to verify that this is a clean machine, and to get
the Service packs updated. I have tried a variety of manual and automatic updates, but cannot get SP 1-1
to update. Right now I have a yellow shield on my toolbar and it says,
Windows found 6 updates.
Will someone walk me through this?
I am a bit paranoid as my pc is in the shop with a
tenacious rootkit which I suspect will at this point
require a reformat, so I need the netbook to work.