Have a read up on this: http://en.wikipedia.org/wiki/Kibibyte or see below for my 5 AM explanation
The good news is you don't really have a problem
basically, 1 byte of information = 8 bits
1000 of these bytes makes up 1KB (kilobytes - think metric, 1000)
1024 of these makes up 1KiB (kibibytes - binary 2^10)
The problem lies in the fact we have two definitions that are used, and when they're used together, we get differing reported values of storage.
Most HDD manufacturers (Intel in this case) use the 'metric' definition; i.e. your hard drive is 160GB. That is 160,000,000,000 bytes.
But Windows uses the binary definition (1KiB = 1024 bytes... 1024KiB = 1 MiB... 1024MiB = 1 GiB) so therefore, you need to convert metric to binary, so to speak.
160,000,000,000 (reporte bytes)
1024 (bytes in a KiB)
= 156250000 KiB
divide this by 1024 to get MiB (mebibytes)
divide this by 1024 to get GiB (gibibyte) (how Windows reports your drive):
= 149.0116119384766 GiB
and that's why it appears you've lost space.
I hope this explains it well enough.