Need more info.
First is it a recent Dell. If it is, then ( sit down before you read this next bit) the HDD could be on the way out, there have been MASSIVE problems with Hitachi/IBM HDD's on Dell laptops over the last number of months, to the extent that Dell are now supplying Fujitsu drives as replacements. As an indication, a good friend of mine in Salzburg has had 3 replacement 80 Gb HDDs in less than 6 months, and it's not a quick job to replace them, even with Ghost or similar, and if you have to do a complete reload from CD's, which I nearly did have to do last weekend, plan on a couple of days work!
Hopefully, it's not a Dell.
In that case, several possibilities.
It might have a driver that's hanging, but not being used, such as network, so when it tries to close it down, it never responds.
Could be asware or spyware causing problems, try running a check or using hijack this to see what's happening, links to that package are all over the place here.
Another possibility is that the disc might be getting very close to full, or is in need of a defragment, if the swap file can't expand, or is badly fragmented, that can play hell with the performance.
If there's several anti virus or firewall packages running, another possibility is that you're getting tight on memory, and it's constantly swapping stuff in and out of "real" memory. Some of the Anti Virus packages ( especially Norton) are real memory hogs, and they've also grown big time in recent months due to all the new garbage that's coming down the line.
Not sure I can suggest anything else till we know a bit more about the configuration and what's running.