Disc space of C drive of my PC is dec...

let me know u 1st, that don't c: or ur system root as ur primary data storage drive..itz a bad practice..

this is a case of a virus/trojan or software is responsible..

check the following:..
1)this can also b a symptom of faulty/defective of RAM..pen clean/removecarbon& or oxide formed on the teeth with an eraser n resitit..[it'll b more prominent, that if itz cause by ram or not,..if duringnormal operation, a Blue Screen appears/Memory Dump]
though, sometimes itz also caused by the m/b..

2)remove/cleandust neatly..dust is non-uniform in nature n may cause jam/shortcircuitry/excess resistance n many more..resulting an increase in heat..

3)open the CPU Heat-Sink with fan out of the socket..put some "HeatSink Compound" (available in local electronics chip level sparemarket)between the cpu/processor and the CPU Heat Sink ***fan..resit/fit it tightly in position..

4) see if ur hdd is noisyor emitting any 'ticking' noise (like 'tuk tuk' or 'kirr kirr'), if sour hdd is likely to b replaced..

[optional] 5) reinstall os..
(to takedata backups u gotta connect the hdd to another pc/laptop, forsuch cases a hdd-case is available in the local spare market, it'llmake behave ur hdd to act as an external removable drive)..

6) hva licensed and UPDATED antivirus installed (i prefer avg/kaspersky,remember, without updation a full version antivirus is useless)..

7) download, install n run microsoft kb890830..http://www.microsoft.com/downloads/details.aspx?FamilyId=AD724AE0-E72D-4F54-9AB3-75B8EB148356&displaylang=en..

8) download n run Startup Control Panel..http://download.cnet.com/Startup-Control-Panel/3000-2094_4-10039330.html..'removeall the unnecessary startup items from here..(if u can't find whatto remove,post me/us in ur comment with all the items listed in eachn every tabs herein)..

9) remove all unnecessary software from add/remove programs..
download, install n run 'Advanced System Care Pro'..http://download.cnet.com/Advanced-SystemCare-Pro/3000-2086_4-10399324.html..

10) disablethe following: a) 'Automatically Restart' right click on mycomputer->properties->advanced->startup n recovery->(uncheckon the) Automatically Restart; b)'Error Reporting' right click on mycomputer->properties->advanced->error reporting->disableerror reporting->(uncheck on the) 'but notify me when critical erroroccur'..

11) Increase the size of Virtual Memory, Initial: 1440; Maximum: 2880

always better to seek professional advice of ur system admin/vendor....

delete the following files temp and %temp%
on the run menu type it it will open the folder
Also u can use select all from file menu to select the hidden files.