Right before you went to bed, what were you doing, how many programs were running, etc. I suspect its a memory issue, because you only have 2GB, and many programs can cause you to start using it all. At that point, the system will start paging/caching hard disk space, which will slow down your system.
To find out more, it would be best for you to determine what programs you have open when it happens. Even checking Task Manager (Ctrl-Shift-Esc) for processes using up 100% CPU and "Performance" can give you a good idea of what is going on.
So I recommend making an inventory of the applications that are open when your system begins to lag. At that moment, you should check the Resource Monitor. In Resource Monitor, you are looking for CPU utilization above 60-75%. You want to look at the memory tab and see if you have very little memory available. Anything under 128MB of free memory left and I would think you have a chance to start seeing a massive slow down.
I am in Windows 7 64-bit with Adobe Reader, Word, and a few programs open here and there, and my memory usage is 4.2GB. If I only had 2GB of RAM on my computer, this would start slowing me down. That amount of RAM will make it very difficult for you to multi-task and have multiple applications (and even browser tabs) running at the same time.
You should consider a memory upgrade, if it is possible.
But first you must determine the issue. Examine the Resource Monitor to see. In the area of disk utilization, if your computer is constantly reading/writing over 1MB/s, you may run into trouble. On average, you should not see much more than 0-500KB/s of Disk I/O on most systems.
So with my technique, you are trying to determine if processor utilization, memory utilization, or hard disk utilization is causing a slow down. In your case, I am telling you that it would most likely be memory consumption that is causing intermittent lag.
Without reviewing logs or monitoring system resources, it would become difficult for you to determine the cause and thereby come to a solution. For example, if you determined memory was a determinate factor, you could upgrade your memory, or reduce the number of programs that open on system startup and remain in the systray.
If I were simply to go by procedure and assumption, and I am being told by an end-user that their system is intermittently lagging, that they are running Windows 7, and that they have a maximum of 2GB of RAM available, the answer is, almost definately, that more memory is required. Windows 7 only requires 1GB of RAM in 32-bit, and 2GB in 64-bit, but these are the minimum requirements for the operating system to run at all. You want to have at least some room for multiple applications. If you cannot conduct an assessment when the system slows down, I would hypothesize that it is being caused by a lack of RAM.