I have 40 gb in c drive but it says low disk space even i have noting in c drive and if i move huge files to d drives and look at the free space after next restart...still same pop message occurs. Infacts i have noting in c drive except the install programms and os.

could u please look this problem and tell me a good suggestion.
I currently using avira antivir antivirus "free version"
and what do u suggest me to use.

u havent mentioned how much ur space is occupied by ur installed programs.
Anyway assuming ur installed program havent occupied nearly the full space,
The solution may be:

• Temporary files on ur computer may be large enough, so delete the temp files manually. For this right click on the c drive, goto properties and then click on disk clean up. then a window will appear showing u all the temp files used up.Now goto next tab (more option) and in 'system restore' click on 'clean up'. Then finally click ok of the main window.
  
• you can use tune up utilities to remove such temporary files and errors too.
  
• Without seeing ur running process we cannot say the problem in computer is due to virus or some spyware anyway I suggest u to use Eset smart security.
  
• To log ur computer running processes u can use 'hijackthis' and post it here.