Malware attacking Windows computers has become such a big issue that Microsoft itself is suggesting the only solution might be a complete hard drive wipe and OS re-install.
Usually I’d say move to another OS, but in this case I won’t. Anyone who knows their Windows machine well already realizes that a wipe/re-birthing every six months to a year is the only way to keep the machine from slowing to a crawl anyway.
The process which I once endured twice-yearly took roughly four to six hours, depending on apps and the availability of hard wired internet access. I suggest wiping clean by deleting partitions and reformatting. Then install operating system and service packs first, base applications like Office next, and then firewall/anti-virus/anti-spyware. Go after third-party apps to wind things up.
Brew some coffee before you start, and get some nice music or a movie playing. It is a pretty boring, often non-interactive process, so you’ll want some distractions.