What part of a software slows down a system?

[sujay]

As far as I know,
If that includes any startup items, processes, drivers, browser addons and occupies large space. It may also cause registry and disk fragmentation.
If a software do not have any such effects, for example a media player, a pdf splitter, can it slow down a PC (when its not running)?
Suppose I have done registry and disk defragmentation after software installation.

[Alboguy]

Based on my experience I think that obsolete registry entries in the registry don't slow down a computer. Cleaning those entries would make the PC...let's say...more responsive. You forgot to mention the services. Services occupy an inconsiderable amount of memory but if you have too many of them they might slow down booting and may create compatibility issues between each other. Media Players and PDF splitters usually don't create services, more complex applications do ex: security softwares, adobe products et.

[Boyfriend]

I will classify software slowdown like this: HDD sector space (size and location), Additional/Bundled features/software, Processes (I/O load), Services, Registry/Startup Entries, Drivers, and compatibility.

Optimize each step to get real speed of software. I have just completed my system optimizations (especially HDD) and voila, it is superfast with less than 10 % CPU load and 50 % RAM, while Google Chrome (11 extensions) is opened with six tabs.

[hellnoire]

The more libraries that need to be installed, reconfigured, recreated... those will also slow your system down. Poorly written code....that's a huge one right there.

[leofelix]

two resident antivirus at once, for instance:-P

[JayCub]

Leo and Paul made valid points conflicting programs and poorly written code, cleaning the reg by backing it up first removing obsolete entries windows searches for that are nolonger valid and defrag gives a small performance boost, after installing and uninstalling lots of software, i can see response gains...
Looking at all the Overclocking forums of late badly written software is going to be the main bottleneck, taking into acount good house keeping, such as disk defrag.. hopefully this will also be a software service no-longer needed in the future if SSD is mainstream new intel cores and good ram make will i hope will make a great compensation regarding the few bottlenecks i haven't solved...

[princeaniket]

i m agree with what albo has said ..
some of the programs like " Adobe products " etc having tendency to alter the startup entries and hence on boot they start interfering in the startup causing PC to slow down .. case is with windows live messenger/Google talk/skype.. although these three causes less time but problem is there also..
i would suggest to read an experiment by " Tune Up " where they installed 200 progrms and then uninstalled it .. after these changes there was a difference caused by left over entries and unused registry..

[safeguy]

Anything that you see in the built-in Task Manager and it's counterparts (Process Explorer, Process Hacker, etc) columns can slow down your PC...

Start-up entries can slowdown the start-up process if the startup items don't 'queue for their turns' properly and you've got too many wanting to start at the same time. Remove those that you don't need and set the less important ones start at a later time (using the built-in services.msc or with another utility)
Services...leave the built-in Windows services as it is if you're not sure. If in doubt, forget about touching it.
RAM usage can slowdown a machine which doesn't have sufficient RAM for example.
High CPU usage can slowdown a machine which doesn't have a fast CPU for example.
I/O read writes can slow down a machine especially when multi-tasking.

User profile space can also slow down your machine. It's wise to set your data to a different partition from the OS.

Registry cleaning doesn't speed up the PC. It's main purpose is to help clean up leftover keys that are no longer used and is meant to prevent problems. (I can't remember exact technical details) Unfortunately, most registry cleaners are marketed otherwise and creates more problems than what they are worth.

If you are in need of a LOT of programs, it's wise to install them to a different partition too if they are not really important and it's function wouldn't be affected by placing it in a different drive. Better option is to make use of portable programs instead for those programs that you rarely need but find useful.

Place your ISO files on a different drive from C:// would help too.

De-fragmentation is good but don't overdo it. De-fragmenting daily is a bad idea as you are thrashing your PC by the constant moving of files. (laymen terms...I suck at explaining things technically)