I would reccomend creating a system repair disk, or disk image after installing an OS. THese days companies do not often include one.
The repair disk can solve issues preventing start up, drivers, and other system problems that can arise from bugs or errors.
A disk image can be used to restore your computer in it's entirety ( it is like a photograph of your system at the time it is created ).
Create one or both after installing and updating the OS as well as any applications you desire ( the disk image is a large file and will require more than one dvd easily, installing applications will of course increase the volume so you may want to just image your clean OS install )
FInd the create system repair disk link on your comp like this:
Start-All Programs-Maintenance-Create System Repair Disk [enter] ...insert dvd in drive, click create.
I recently repaired a computer with a corrupted registry - it wouldn't even let me run a cd to try either option. THis is when a large jump drive or portable HD is handy - you can perform the same tasks from there.
For anti-virus, I have come to respect Microsoft Security Essentials - I know a well versed man who has been trying to break it for about a year by intentionally perusing sites he knows he has gotten bugs from in the past ( he likes this kind of thing too lol ) Not to mention my son, who has had a habit of needing to reinstall because of bugs at least once or twice a year, he hasn't had an issue either! ( You don't know him, but you'd be impressed with MSE if you did hehe )
As for maintenance, I suggest CCleaner, Piriform, the company behind CCleaner, does offer a defrag and a couple other nice little applications to assist with maintenance - http://www.Piriform.com
CCleaner can perform registry cleaning and defragging also, which is another computer slowing area. Everything you install creates registry keys...these are not all deleted when you uninstall a thing. Cleaning and defragging your registry can greatly imrove a slow computer's peformance ( I'm convinced this is a little service your computer guy won't tell you he uses to improve your comp's speed because you wouldn't need him - most other performance enhancing actions are well known ).
You see, every instruction your computer receives, from the press of a key to the shutdown of the system, goes to the registry to find out what the heck that is and what to do with the instruction, so you can see if your registry is bloated, this can slow your system.
Do use the option to backup your registry before allowing the application to clean it.
A couple good resources are http://www.WIndowsSecrets.com and http://www.MaximumPC.com, which has a pdf archive dating back to 2003 that is downloadable, indexable and thus easily searched on your computer.
One other note: should you have a problem arise and download a reapir disk from a torrent site, be aware that all the proper drivers may not be included ( a friend once gave me a disk with every version of 7 on it - 32 and 64 bit, had he used it he would have realized it had no drivers included and so was a waste of his dl time and disks he printed it on for others lol ).
Even if drivers are there, they may not be the right ones for your comp, you may need to locate your NIC( Network Interface Card - allows communication with other computers, thus the internet ) driver and install it. I have seen this often when the user hasn't created a system repair or image disk.
You also will need a crack if you choose to dl an OS, make sure one is included unless you know the Windows OS version coding in your OS version's key, which is required to authenticate windows.