I am having the same issue.
System is:
MSI-6315
PIII 1GHz coppermine
512MB RAM
ATI Radeon 9000 Pro 128MB
Intel Application Accelerator installed
Maxtor Diamondmax 80GB ATA-100 (drive running at UDMA5)
MSI CR52 (52/32/52) (drive running at MW-DMA2)
Windows XP HE
Nero Express 6.3.0.3
Memorex CD-R 80Min 48X Multi speed
Shows that 30.4X is the maximum, then says the source data is too slow and burns at 24X
Under the Cache tab the tested drive speed is 57MB/s
I have the disk cache set at the maximum of 80MB
Also ... the FAQs on Nero site said to uncheck Buffer underrun protection, but I don't have the option of disabling it. Maybe MSI has a program for tweaking that?