"Previous" Button has disappeared

Question

I'm on Windows XP Pro, 2 Gig RAM, Raid 1 (2 36GB Raptors), 500 GB in HD on two drives, Externals total 1 Tb. My Lightroom is the trial version downloaded last week (May 18th or thereabouts). I am a professional photographer. www.twmeyer.com.
Here's my problem:
the "previous" button is not showing up in the "Develop" module, and even though the help files say
"In the Develop module, click the Previous button to the right of the toolbar, or choose Develop > Paste Settings From Previous.
there is no "Previous" button, and the Help menu further states
"When multiple photos are selected in the Filmstrip, the Previous button in the Develop module turns into the Sync button."
But I can't tell how to select (aside from just clicking on an image), if multiples are selected, or how to de-select (aside from clicking on a different image). Just above the Filmstrip, in the area where the filter settings are, it is indicated that I am in a particular collection, that there are 709 photos in that collection and they are all selected: "709 photos / 709 selected".
I suspect this "selection" is what's causing the problem, as in "When multiple photos are selected in the Filmstrip, the Previous button in the Develop module turns into the Sync button"... which is exactly what I'm looking at... The coveted "Previous" button is now the unattractive "Sync" button.
Any other ideas?.
I don't want to have to Cnrtl>Alt>V my way through hundreds... thousands of images. The advantage of the previous button is that each image is not exactly like the next, but the next is frequently very similar to the previous one (within a relatively contiguous set of moments), so I can correct the first in a series of images in a complex manner, move to the next image and click "Previous" and then apply the minor tweak necessary. This, rather than go through all the different settings on every image.
Since it is, from one photo to the next, an incremental and shifting change (like getting closer to a window, or perhaps a cloud goes by that window, or moving deeper into a tungsten lit room) it doesn't make sense to apply any one set of corrections to all the photos in these "real world" images (not studio lit). I want to fluidly move through the collection bumping exposure on this one, apply those changes to the next image and maybe bump the WB on it, and so on.
I want the "darned" :^) "Previous" button!... Help!... t

Answer 1

and solved. It was some sort of locked on selection. I'm not sure the problem started, but it may be connected to the library being reconfigured. It (the Library) was on the C drive but the images were on an external that is turned on and off as needed. This may have caused some conflict that I haven't the tech-head to noodle out.
I just got another 500G External drive that I plan to dedicate to the Lightroom Library and all new images for editing. Does this mean I have to delete and re-import again? or can I move it from my C drive otherwise, somehow? I plan to generate sidecars with all new imports, hoping that I will then be able to move the images to other externals (and optical disks) for archiving, and carry them to my studio computer with LR data intact. This archiving and pass through will be of both original NEF files and derivative PSDs and JPGs. Workable?
I will be getting (and reading) Martin Evening's Lightroom Book before taking any more significant actions. It was in his PDF download (and from Ellis V) that I discovered the idea of locating the Library in an external or secondary HD, rather than in the default location on my precious C drive.
Thanks again for all who tipped in on this issue and who might address these secondary issues (or should they be another thread?)... t