Check your antivirus program. I know programs such as Norton and AVG Give Access Denied until they have finished scanning the newly inputed devices.
With the CD/DVD drive make sure under device manager that the driver has not become corrupted.
A friend is running Windows XP SP3 on an Acer desktop. In the last few days, she started getting "Access Denied" when trying to use an external USB drive or flash drive. She also gets it with the CD drive when trying to read/write a CD. Any suggestions would be welcome.