BOAC,
To a large extend I've got no issues with what SD says. It's simply not feasible for anything to install itself .... and viruses/trojans/spyware all rely on installation in order for them to do their deeds.
Of course, there are one or two possible exceptions ... such as a buffer overflow attack. However new features such as DEP and ASLR (given we're talking about Windows here) have made significant moves in the right direction to mitigate remaining risks.
In any event. All computer users, irrespective of operating system, should seek to do their day to day work on the principle of "least privilege" and then escalate privilieges only as and when necessary.
If you remain truly paranoid, take a look at Faronics Deep Freeze ...... one reboot and you're back to a clean state. It's been thoroughly field tested in schools, libraries and ohter "high risk" environments ...... so by all accounts it works......
Faronics Deep Freeze Windows Editions - ABSOLUTE System Integrity