Disable onboard sound & driver - You should only have 1 soundcard/interface enabled at once, so yes. If you have an onboard soundcad, disable it through the Windows Device Manager.
Right-Click "My Computer", click "Properties". Click the "Hardware" tab and select Device Manager. This is for Windows XP, might be a little different on a different version of Windows.
Download all Windows Updates - I only have up to the SP2 of Windows. I haven't downloaded any updates past that. It's good to download/install the Windows updates, but only in rare cases will it effect your soundcard/hardware. Unless Windows has an update for your soundcard driver. Unless it's onboard/popular, it probably won't.
Download all chipset updates - This can be usually done through your BIOS manufacturer website (ASUS/ECS, etc.). I haven't updated mine in years.
Tried soundcard in all PCI slots - Only if you have problems installing it. If you do, try another PCI slot.
Disable anti-virus software - I personally don't run anti-virus software. If you know what you're doing/downloading, you won't have to worry about this. BUt it is important to disable it while installing the hardware drivers.