Dust on your lens can cause a DVD drive to not work as well as it could.
Cleaning the outer lens is an option but there are problems with this
approach. First of all, the laser units have a lot more than one spot that
collects dust and you can't get to all of them without entirely disassembling
and ruining the laser unit itself. Secondly, ALL lenses will have SOME dust on
them just because they're an exposed part. If your DVD drive isn't working
because of a few dust particles, it's an indication that it's on it's way out
anyway.

NOTE: Don't blow compressed air into your Xbox or use a vacuum cleaner to
try to remove the offending dust. It won't help, and it will probably make it
worse by stirring up what dust is already in there.  

There are several different brands of CD player and DVD player lens
cleaners. They're compact discs with little tiny brushes on them that are
supposed to knock the dust off when you put the CD in and it spins around.  
Neither I nor anyone I know have had much success with them as far as
Xboxes are concerned, and in several cases they appear to have knocked the
lens out of alignment and ruined them altogether. If you've given up on your
existing Xbox drive and happen to have a cleaning disc laying around go
ahead and give it a try. It might actually work, and if not, you're just back to
where you started. I wouldn't suggest you go out and buy one for this
purpose though, as you'll probably be wasting your money.

The only way I know of to safely and effectively clean your lens is to do it by
hand, preferably with a good quality "make-up-sponge-on-a-stick" type
applicator and some 91% isopropyl alcohol or proper lens cleaner. Q-tips are
OK too but I find they tend to leave little strands of cotton behind and it only
takes one to mess everything up. If you use a Q-tip just inspect VERY closely
to make sure there's no cotton strands left behind.

This is a very delicate procedure as applying any pressure at all to the lens
could mess up the alignment and ruin the laser, they're very susceptible to
static discharge problems too. It's really a job for someone that knows what
they're doing. I do it for my local customers for $20 but there's really no cost
effective way for me to help my long distance customers with this. Your best
bet would be to try to find someone in your local area that will do it for a
reasonable price. Much more than $20 isn't reasonable. Keep in mind that it
will take them about 15 minutes and requires no special parts or tools. If
they want more than that you might as well replace the whole DVD drive
yourself and know that it will be good for a couple years. Like I said above, if
a few dust particles are causing a problem the laser probably isn't going to
last much longer anyway. If your Xbox has been exposed to some seriously
MAJOR dust though, like installing sheetrock or something, cleaning the lens
might be all it needs. You'll want to clean the entire drive though otherwise
the dust will just settle on the laser again.
Thanks Lynn.  I
received the dvd drive
and installed it last
night.  My xbox is
alive again!  Another
satisfied customer.  
Take care and happy
holidays.

-John