The points where you can 'read' usage data differ in how they measure and report data. Often counting cached and swapped data as 'in memory'. While this is technically accurate from a software perspective, it does not directly relate to actual usage of the physical DIMMS.
The easiest way to get an overview of what is going on is to open Resource Monitor - There is a tab for Memory usage which is pretty self explanatory:
Hardware Reserved (gray) This is physical memory that is set aside by the BIOS and other hardware drivers, and can not be used for anything else.
In Use (green) In active use by Windows, running processes, or by device drivers. This is the number that matters: If you consistently fill this green bar across the entire length of the graph, then you are trying to push your physical RAM beyond its capacity. Consider upgrades if that is the case.
Modified (orange) This represents pages of memory that can be used by other programs but would have to be written to the page file before they can be reused.
Standby (blue) This is the amount of memory that Windows 7 is using for cacheing ('Superfetch', etc). It tries very hard to keep this as full as possible. If you start a new process that needs memory, the lowest-priority pages on this list are discarded and made available to the new process. This is transparent to the user and to applications, and should not interfere at all with performance.
Free (light blue) As I pointed out above, Windows Vista and 7 try very hard to avoid leaving any memory completely free. Over time, Windows will use idle cycles to slowly fill this up with items it thinks you may use. This is at a very low priority and will not interfere with system performance.
When you say "RAM is completely full", what are you looking at that tells you this?
Probably the best single indicator is to open Task Manager (Ctrl-Shift-Esc), click the "Performance" tab, and look at the "Memory" graph. If that shows an excessive amount of memory in use, then Scotteq's suggestion of looking at Resource Monitor is an excellent one.
Just on the face of your explanation, I suspect that what's happened is that Windows has gradually moved a lot of stuff out of memory and used it to cache your downloaded files. When you don't use your system for 6 to 8 hours the system starts to optimize it for background stuff like virus scans. But it should start responding normally once you've "touched" all your active programs and they get paged back into memory.
One thing you COULD try, assuming that you don't NEED more than 8GB for the programs you're running, is to eliminate the page file. That would prevent programs from being paged out of memory in the first place. But you should only do this after confirming that your memory usage (as shown in Task Manager) never gets above, say, 6GB of your 8GB of physical memory EVEN when you're running the biggest, most bad-ass combination of stuff you ever run at the same time.
OK, so ignore my comments caching above, from your screenshot that's not the issue. If it was then the bar would have been mostly blue.
It looks like your process list is sorted by descending committed bytes, but there's nothing on the visible part of the list that accounts for all the space that's used - it's very odd. Can you click on the "Working set" column heading to sort the list by working set size and see if there's any process that has a really large working set? The number is in KBytes, so to use several GBytes you'd have to see a number that's in the millions.
OK this is the latest screenshot i could get. Notification flag says Scheduled Scan. I am running no where near 300 processes, more like around 100 max and i frequently go through my startup config and remove unnecessary files, at least once a month. Windows loads very fast, almost as good as a fresh install.
I still think this scheduled scan has a lot to do with it. It was running fine for a few hours while scanning, and then the ram filled all the way up out of nowhere. i use the cpu meter gadget to monitor my ram usage, and it goes up as high as 95%, and no lower than 85% when this occurs.
anyway, here's the screenshot. let me know what you think, i am at a loss.
Still seeing nothing in your process list that looks like it could account for the space. That's actually consistent with a power manager handle leak in the kernel as described in the patch I linked to.
What are your power options? You might try changing them (for example, disabling sleep mode or changing the minimum power levels or something) so see if it really is related to the power manager.