Hopefully, you don't have the hard-drive burn virus. It's a virus that spins the hard drive up and read/writes to the same physical spot on the drive. After 30/40 minutes of this, the higher RPM drives (7200-10k) can catch on fire, especially in a poorly vented case. I know it exists, because one of my client's machines caught on fire from it

.
It's extremely easy to detect with AV software, or from the fact that the hard drive is constantly grinding away. My client says that she saw one of those pop-up spam things that comes through windows msgsys.exe, and then a little while later it started. However, she let me check her web-email from my computer, and Norton caught *a* virus immediately.
I doubt this is what you have, but keeping the case cool for now would generally be a good idea. Even if it's not this virus, the added heat from the hard drive isn't good for the processor, and DELL computers are well known to be "quiet", aka no case fans...