Sometimes computer programs get out of control and start using up all the machine's CPU time, slowing the computer down for everyone else who's trying to use it. This most often happens when firefox or acroread (Acrobat Reader) crashes.
You can check whether you have any runaway processes by typing "top", which which will give you a regularly updated display of the processes, beginning with the ones using the most CPU. Press "q" to exit top. Here is an example of the output from the "top" command:
top - 14:56:44 up 4 days, 2:33, 3 users, load average: 0.04, 0.04, 0.01
Tasks: 127 total, 1 running, 126 sleeping, 0 stopped, 0 zombie
Cpu(s): 0.5% us, 0.3% sy, 0.0% ni, 99.2% id, 0.0% wa, 0.0% hi, 0.0% si
Mem: 2056828k total, 1768856k used, 287972k free, 267712k buffers
Swap: 2031608k total, 0k used, 2031608k free, 960008k cached
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
4909 root 15 0 168m 36m 5936 S 1 1.8 10:31.86 X
16106 eva 15 0 35920 13m 8472 S 0 0.7 0:00.64 gnome-terminal
1 root 16 0 2524 552 472 S 0 0.0 0:00.70 init
2 root RT 0 0 0 0 S 0 0.0 0:00.08 migration/0
3 root 34 19 0 0 0 S 0 0.0 0:00.04 ksoftirqd/0
4 root RT 0 0 0 0 S 0 0.0 0:00.02 migration/1
5 root 34 19 0 0 0 S 0 0.0 0:00.00 ksoftirqd/1
6 root 5 -10 0 0 0 S 0 0.0 0:00.38 events/0
7 root 5 -10 0 0 0 S 0 0.0 0:00.32 events/1
8 root 6 -10 0 0 0 S 0 0.0 0:00.00 khelper
9 root 15 -10 0 0 0 S 0 0.0 0:00.00 kacpid
46 root 5 -10 0 0 0 S 0 0.0 0:00.00 kblockd/0
47 root 5 -10 0 0 0 S 0 0.0 0:00.00 kblockd/1
48 root 15 0 0 0 0 S 0 0.0 0:00.00 khubd
65 root 20 0 0 0 0 S 0 0.0 0:00.00 pdflush
66 root 15 0 0 0 0 S 0 0.0 0:00.64 pdflush
67 root 25 0 0 0 0 S 0 0.0 0:00.00 kswapd0
On this machine, nothing is using very much CPU. You can tell this from the line where it says ``99.2% idle'', or from the very low numbers in the ``%CPU'' column.
If you see any runaway processes of yours at the top of the list and eating lots of CPU, you can get rid of them by typing ``kill PID'', without the quotation marks and replacing ``PID'' by the number in the first column. You may need to repeat the kill command or use ``kill -9 PID'', which is a more emphatic kill and should work even on processes which are too mixed up to respond to ``kill PID''. You won't be able to kill other people's processes by accident.
If it is your web browser which ran away, then you may need to locate and remove any leftover ``lock'' files before the browser will start working again. Search the .mozilla directory for files with ``lock'' in their name.