System properties reports wrong RAM size

[Tellos Athenaios]

Or you got back a copy of a your disk image on a brand new laptop with different specs?

Things you can do:
(a) Use a memtest program and boot that (run it outside of Windows or any other OS). This should report among other things how much RAM you actually have (and whether any of it is damaged/inaccessible). [Note that if a large-ish range of RAM comes out damaged/inaccessible it may simply be reserved by the BIOS for itself and not damaged at all: your memtest may simply not recognize this.] Google one. If the RAM is damaged, complain at customer support or buy new sticks (not very useful if you are unable to install the sticks yourself though).
(b) Open the case and count the sticks. Might void the warranty if any still applies. Only very useful if you knew how many sticks were/should be inside in the first place (but you may be able to find out from where you bought it). If the sticks don't add up, buy more RAM and install it (you should be able to install them if you could open the machine and count the old sticks in the first place) or get customer support to do it for you.
(c) Consider Windows to be unreliable and reinstall Windows. I doubt the problem would go away, but then again I'd have doubted more things about Windows.
(d) Consider Windows to be unreliable but reinstalling Windows too much of a hassle: download a LiveCD/LiveDVD/LiveUSB image of some other OS, burn it to a disc and reboot your PC with the disc inside (note that you will want to burn at the slowest speed possible [less chance of error] and also let your burning program verify the written data).
When you have booted into your live OS, find out your hardware specs according to that OS and see if it differs from what Windows reports. If anything differs you will still want to do (c).
(e) Might work; but it relies on the ability of a damaged OS to repair itself: make a recovery disc of your Windows then use the recovery disc to repair/restore Windows to factory defaults.

Note that (c) is bound to and (e) might result in data loss.

[Husar]

Things you can do:
(a) Use a memtest program and boot that (run it outside of Windows or any other OS). This should report among other things how much RAM you actually have (and whether any of it is damaged/inaccessible).

I just happened to run memtest 86+ lately and it showed my 4GB DDR2 800MHz as DDR1 500MHz so not sure about that.

If there are a whole 2GB missing first thing I'd do is look for some hatch underneath where the RAM can be replaced and see whether they didn't take some of it out.
If that's not possible for any reason (no hatch, lack of knowledge, fear for loss of guarantee) I'd have a look at the BIOS startup (usually shows amount of available RAM, if one can enable advanced startup test it should count/test through all of it), then there are those system diagnostics tools like CPU-Z which shows you what stick you have in which slot etc. Though I guess it gets that from the BIOS, so not sure about the reliability of that.

If you're in doubt, go back to the store and tell them about your missing 2GB and that you'd like them back.