When I power on my PC, the fans will start as normal for around 3 seconds, before shutting down. This happens twice, before it will start booting up normally.
Something similar also happens after attempting to wake the machine from sleep. The fans will start up and shut down twice. The exception here is thought, that it will never go into the OS (win7) and the Screen remains blank. The fans will just keep spinning (at rather high frequency), but nothing happens. The only way to get out of this, is to completely shut down and restart the machine.
and this started happening after I disassembled and reassembled my pc for cleaning purpose.
Not knowing how far you disassemble the system, is it possible the CPU fan is not correctly plugged into the proper connection? Also did you remove the CPU cooler and if so did you reapply thermal compound properly? Are the RAM modules fully seated and installed correctly? Have you tried to reset the BIOS or did you replace or reseat the CMOS battery? Also make sure the drives are plugged in correctly and that the boot sequence is correct in the BIOS (Windows Boot Manager should be first). Did you remove the CPU and if so make sure none of the pins in the socket are bent.