I still do a lot of unofficial consulting on PC support, and folks have asked me for the Cole's Notes version of what basic software they need to surf safely and keep their machine running in top form.
So - for hardware - I remember a article from HP about spring cleaning for the hard drive which summarizes the best tweaking for a Windows based machine.
HP Hard drive maintenance basics Spring cleaning time for your hard drive. Not such a bad idea. I've always maintained all my machines by running DEFRAG and Disk Cleanup on a monthly basis and CHKDSK, anti-virus and Spyware removal on a weekly basis. Being a belt and suspenders type of IT guy - I use Ad-ware and Spybot Search and Destroy to get rid of the offending stuff. It's incredible the amount of faldercarb browsers pick up from advertising. So I use some free software to keep things up to date.
First off - Ad-aware - it can be downloaded from Download.com or get the current copy of Ad-aware www..lavasoftusa.com directly. Spybot Search and Destroy - Spybot is from downloads.com as well - but you can also get it direct from www.safer-networking.org. Finally, here's the Anti-virus I recommend - AVG Anti-virus from www.grissoft.com. There are a couple of versions - one with a built in firewall for use when you have ADSL or cable modem connections to the internet, and commercial versions. But one of the nicest ideas they have is a single home user version that is absolutely free. I keep a copy of the free version for fixing up home machines that don't have any sort of Anti-virus, and protecting them until they buy something to do ongoing protection. Get AVG for Free
As to a firewall - on my own machines I have a very nice hardware router at home from Linksys - with the latest patches for the software and all the Windows patches up to date. That's another of my little tweaks - once a week I run the Microsoft update on all my machines to make sure all the patches are current.
For stand-alone machines - I recommend Zone Alarm Pro from the folks at Zone labs. And like the folks at Grisoft - they have a free version that is a basic firewall, which I find is great to put into laptops with Wi-Fi connections - since you never know where your connection is coming from. The Pro version does quite a bit more as well. Check out "What are My Security Needs" on their site for a comparison of the Pro vs. Free.