Hmmm... Odd problem... A few things to mention
1. Your audio is Realtek (as you guessed earlier) and it connects to your PCI bus. So it's odd that whatever the problem affects both audio (on PCI bus) plus your USB ports. Of course, nothing is impossible with computers (or Windows!) but are you sure these two problems always occur together? or might they be separate problems?
2. When your audio is not working, the data you provide doesn't seem to point to a driver issue but, just to be sure, when you're having the problem open Device Manager. Look closely at all entries under the USB, System and Sound categories. Do you see any red X's? or yellow icons on any device?
3. Also, when the audio is not working, click Start->Run, enter: services.msc. Scroll to Windows Audio service. Verify if Status=Started and Startup Type= Automatic?
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Oh! I just noticed you're also still running XP SP2!
> Do you run Custom Windows updates? Have you installed all the hardware optional updates
> You should also upgrade to SP3 (but be sure to backup all your stuff. Good to do a backup image of HD in fact). Most SP3 upgrades go smooth, but some don't... so be prepared with bakups, just in case
> Also are you aware MS is dropping XP SP2 support very soon (like sometime this summer)