Recreate the Windows ME screensaver as ASCII
Nothing says nostalgia quite like...

QBasic, 332 bytes

INPUT t,w,h
RANDOMIZE
CLS
1b=INT(RND*4)
d=b
IF b MOD 2THEN c=(b-1)/2*(w-1)+1:r=1+INT(RND*h)ELSE c=1+INT(RND*w):r=b/2*(h-1)+1
WHILE t
LOCATE r,c
m=(b+d)MOD 4
IF b=d THEN x=8.5*m ELSE x=13*m+(1&lt;((b MOD m*3)+m)MOD 5)
?CHR$(179+x);
r=r-(d-1)MOD 2
c=c-(d-2)MOD 2
b=d
d=(4+d+INT(RND*1.25-.125))MOD 4
t=t-1
IF(r&lt;=h)*(c&lt;=w)*r*c=0GOTO 1
WEND


QBasic is the right language for the task because:


Its encoding includes box drawing characters--no need for Unicode
LOCATE allows you to print to any location on the screen, overwriting what was there previously
Microsoft®


Specifics

This is golfed QBasic, written and tested on QB64 with autoformatting turned off. If you type/paste it into the actual QBasic IDE, it will add a bunch of spaces and expand ? into PRINT, but it should run exactly the same.

The program inputs three comma-separated values: ticks, width, and height. It then asks for a random-number seed. (If this behavior isn't acceptable, change the second line to RANDOMIZE TIMER for +6 bytes.) Finally, it draws the pipes to the screen.

The maximum dimensions that can be entered are 80 (width) by 25 (height). Giving a height of 25 will result in the bottom row getting cut off when QBasic says "Press any key to continue."

How?

TL;DR: A lot of math.

The current row and column are r and c; the current direction is d and the previous direction is b. Direction values 0-3 are down, right, up, left. Arithmetic translates those into the correct step values for r and c, as well as the correct edge coordinates to start on.

The box drawing characters │┐└─┘┌ are code points 179, 191, 192, 196, 217, and 218 in QBasic. Those appear pretty random, but it still used fewer characters to generate the numbers with some (pretty convoluted, I'm-not-sure-even-I-understand-it) math than to do a bunch of conditional statements.

The code for changing direction generates a random number between -0.125 and 1.125 and takes its floor. This gives -1 10% of the time, 0 80% of the time, and 1 10% of the time. We then add this to the current value of d, mod 4. Adding 0 keeps the current direction; adding +/-1 makes a turn.

As for control flow, the WHILE t ... WEND is the main loop; the section before it, starting with line number 1 (1b=INT(RND*4)), restarts the pipe at a random edge. Whenever r and c are outside the window, we GOTO 1.

Show me the GIF!

Here you go:



This was generated by a somewhat ungolfed version with animation, color, and an automatic random seed:

INPUT t, w, h
RANDOMIZE TIMER
CLS

restart:
' Calculate an edge to start from

b = INT(RND * 4)
'0: top edge (moving down)
'1: left edge (moving right)
'2: bottom edge (moving up)
'3: right edge (moving left)
d = b

' Calculate column and row for a random point on that edge
IF b MOD 2 THEN
    c = (b - 1) / 2 * (w - 1) + 1
    r = 1 + INT(RND * h)
ELSE
    c = 1 + INT(RND * w)
    r = b / 2 * (h - 1) + 1
END IF
COLOR INT(RND * 15) + 1

WHILE t
    ' Mathemagic to generate the correct box-drawing character
    m = (b + d) MOD 4
    IF b = d THEN
        x = 17 * m / 2
    ELSE
        x = 13 * m + (1 &lt; ((b MOD m * 3) + m) MOD 5)
    END IF
    LOCATE r, c
    PRINT CHR$(179 + x);

    ' Update row and column
    r = r - (d - 1) MOD 2
    c = c - (d - 2) MOD 2
    ' Generate new direction (10% turn one way, 10% turn the other way,
    ' 80% go straight)
    b = d
    d = (4 + d + INT(RND * 1.25 - .125)) MOD 4

    ' Pause
    z = TIMER
    WHILE TIMER &lt; z + 0.01
        IF z &gt; TIMER THEN z = z - 86400
    WEND

    t = t - 1
    IF r &gt; h OR c &gt; w OR r = 0 OR c = 0 THEN GOTO restart
WEND