The fact that it's intermittent makes it difficult to diagnose. Here are some suggestions for when the drive NEVER appears:
1. Uninstall the drive through Device Manager.
Restart computer. The drive will be automatically reinstalled.
or…
2. Delete Upper & Lower Filters
http://support.microsoft.com/kb/982116
Restart computer.
or…
3. Download, and run Restore Missing CD Drive patch
Double click on cdgone.zip to unzip it.
Right click on cdgone.reg, click Merge.
Accept registry merge.
Restart computer.
or…
4. Go to Device Manager, click a "+" sign next to IDE ATA/ATAPI Controllers.
You'll see two items:
- ATA Channel0 (or Primary Channel)
- ATA Channel1 (or Secondary Channel)
Right click on each of them, and click Uninstall. Confirm.
Restart Windows. They'll be automatically reinstalled.