Before installing, make sure you update your hardware and check your software to see if there are any updates. Most software will work with SP1 but there are a few cases where an update would be required.
Also, update your antivirus, anti-spyware and run scans (in SAFE MODE if possible) to make sure you don't have any infections. Get cleaned by the professionals here at Bleeping Computer if you need to.
Make sure you close all programs and DISABLE all your realtime protection before installing SP1. Just keep the Windows Firewall on....that should keep you protected enough until the computer reboots. If you're working behind a hardware firewall (ie: a router) then even better.
Turn off your screensaver also.
The install can take anywhere from 30 minutes to over 5 hours!! It all depends on what's on your system. Most of the install will occur outside of Windows.
Do not turn off your computer while SP1 is installing. At times, your computer may appear to hang for a while and even your Hard drive may go quiet. Don't worry...this is normal. I would suggest that you go and do something else while SP1 is installing due to the length of time it takes.
The comp can reboot once, twice or more during the install. Again, do NOT interupt it.
Once you are back at your desktop, the install is over and SP1 should be on your computer.
Good luck.