Updates are worth downloading, because most of them plug bugs and virus back doors in Micro$oft software.
But if you have a machine used for something critical (e.g. Accounts, flight planning while travelling, etc) then you should update it fully and then disable all updates on it, because updates have been known to break things. For example WinXP SP3 trashed the hard drives of thousands of PCs around the world - including a pricey Thinkpad laptop of mine. But then you have to enforce a strict internet usage policy on that machine (no kids, no general browsing of dodgy websites, no M$ email or browser software).
I run XP SP3 normally and use Kaspersky antivirus - the only AV software which has never given me trouble, across a fair range of machines.
You must disable all updates (which is actually quite tricky, given how many apps do this) if you use mobile internet, especially outside the UK
