I've been able to replicate what I described above twice. The first time I installed and uninstalled the Support Pack, and restarted, Windows updated the drivers automatically to version 2.18.0013. The 2nd time it didn't, and I installed the 2.18.0013 drivers manually. Both times the sound problem went away, until I restarted, and it came back again, even though it didn't update again after I restarted.
I just looked up the specs of your Dell,
Documentation
and have a good idea as to what is causing the problem. It is the Nvidia nforce 4 chipset, combined with the 4 GB of ram and the X-Fi. That combination was very hit and miss as far as the problem occurring (and happened on both the Intel and AMD versions of that chipset, with some having the problem and some not), which made it a pain trying to determine which was actually to blame. It also isn't a new thing with that combo and has been going on since the cards came out,
nf4 x-fi - Google Search
One thing that most found though was that once they moved to a new computer or motherboard that didn't have that chipset, their sound card started working OK.
Doesn't help much in your case though. You basically have 3 options,
1. Pull the card and use the onboard sound, and save the X-Fi for the next computer.
2. Get another sound card, instead of using the onboard sound.
3. Only use 3 GB of ram.
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