PC configuration-
AMD Athlon II processor
4 GB DDR3 ram
1 GB Nvidia GeForce graphics card
500 GB HDD

I installed Linux Mint inside Windows 7. But the close, minimise and maximise buttons disappear (of every window that is opened) most of the time. After logout it becomes alright but since few days even that is not helping.

When you say "... Inside Windows 7." do you mean in virtualisation software of some kind? If so, then turning off compositing in Mint may help. Actually, it may help even if it's not virtualised but in that case you may miss the compositing for things like wobbly windows and transparency if you like them (assuming Mint has this).

This has happened in my installation of Mint 11 as well. Here is how I fixed it.

Open Control Center
Open CompizConfig Settings Manager
Scroll down to the Effects section and if Window Decoration is not checked, check it. If it is already checked, uncheck it and re-check it.

This should fix it. If it does, make sure and mark this question "Solved"

Good luck

lrfocke

One more thing.
Also turn off the annoying double click on title bar options. I found that besides being annoying it could cause your main problem of the title bar disappearing. Since I turned off any double click option on title bar it has not happened again. To do this open control panel, in the first section "personal", click on the windows icon and under the Titlebar Action setting, set it to "none". For six months not I have not had the title bar disappear on me.

Good luck and let us know if this works on your setup as well.

lrfocke