Thanks for replying.
Everything seems to check out fine, those games don't occupy too much space, and are few in number. Your internet speeds are definitely not a bottleneck. All the above, in combination with your stated specs (Core i5, Radeon 5850 and what I assume a good amount of RAM), would leave out one culprit, your hard drive.
There is no logical reason why Steam would take so long and still grind your HDD 6+ hours after commencement. Only other thing I can think off (in terms of hardware), would be extreme fragmentation of the drive. Consider defragmenting the drive if you have the time to spare, it would also help with the general performance of your OS and other apps.
Also, consider running a chkdsk (start--> cmd ---> right click, run in administrator mode --> type in chkdsk /f ) on the drive too. Best done when you have a lot of time to spare as it'd take hours on a large TB drive. Just to be sure that the drive is healthy.
I forgot to ask one more important question, what sort of antivirus/malware solution are you currently using? (McAfee, Norton, MSE, Avast, etc)
Have you tried temporarily disabling your AV solution before starting up Steam and letting it do its thing?