4GB RAM Installed - 1.75GB Usable

FIX FOR ME - I had the same problem with an acer aspire laptop running windows 7 showed 3gb in windows but only 768mb usable but I thought of the previous answers and thought "why would this start playing up overnight after months of trouble free use with no driver updates?" so I removed the 1 gig stick leaving the 2 gig stick in and there was no POST - replaced the 2 gig with the 1 stick I had removed and post fine DA DA faulty 2 gig stick! - replaced the 2 gb stick and I now have 2.75 usable out of 3 gig
Windows checks the memory for reliability and if it see any problems it will map out the hardware but still show it as installed.

These memory reading are very typical for most lap tops. I have 4gb installed, SysInfo reports total physical as 3.49, that's the 4.0 minus what's used by the gpu. and available is 1.65gb, that means the system uses about 1.8gb as a running OS.

That's sysinfo the original complaint, as I understand it was the windows 7 system screen where it just gives you the basics - mine was reading 3gb (768mb usable) now it reads 3gb (2.75gb usable) - thanks for your input though

I believe that the memory size as reported in the system info utility and in system properties is obtained by reading the identification of the RAM chips. Many utilities do much the same. It takes no account of whether the RAM is functioning or accessible. Total memory as reported in Task Manager has been tested as functional, although only at a very rudimentary level. A reliable memory test takes considerable time.