So there is a swap file existing.
That's a dedicated place of the HDD for the OS to use exclusively for storing data temporary.
Apparently the RAM is not big enough for everything.
The swap file size recommendation according to the Virtual Memory info of System Properties: almost 5 GB.
Alright, so that's the space that is been reserved from my HDD.
Some other facts: the transfer speed of a typical HDD is approx 40 to 90 MB/s, RAM is approx 2 to 8 GB/s fast and a USB device 5 to 30 MB/s.
Looks like the best option would be to extend the RAM, but if you have an 32-bit OS, then 4 GB RAM is your max limit.
For everything more than that you need a 64-bit OS.
Anyway, so since Vista we have this feature called ReadyBoost.
With that we can now use a USB device as additional buffer zone.
I'm using an 8 GB USB stick, but ReadyBoost can only go up to 4 GB, alright.
I haven't succeeded to format it in NTFS but only the FAT format and neither could I assign at least 4 GB but only little more than 1 GB for ReadyBoost.