One day you feel your computer becomes slower. Adding RAM won't help.
There may be many different reasons: one of them is - IDE channel serving your IDE ATA or ATAPI disks is running in PIO mode instead of DMA.
Symptoms: start Process Explorer (made by ex-Sysinternals, nowadays Microsoft guys Russinovitch & Co. http://technet.microsoft.com/en-gb/sysinternals/bb896653.aspx ) and monitor CPU usage. If you see, that "Interrupts" pseudo-task is using significant - over 30% - CPU time, follow the MS Knowledge Base article http://support.microsoft.com/kb/817472.